Okay I didn't realize how long this post was until after I wrote it, but I hope it's of benefit to you and your friend. I'm no great expert so I hope others will chime in where I'm misleading or inaccurate.
I think there may also be a misunderstanding of what bodhisattvas and buddhas and deities like Chenrezig are in Buddhism. In the Western world, we very much tend to apply the materialistic mindset (me in here, you out there) to everything, so we get a Christian god that is separate from us out there intervening in the world (from where??) if you ask nicely. Then naturally, this view gets carried over to Buddhist deities and spirits.
We must start from our relativistic viewpoint, since materialism is the condition of our times. So we recognize that merely accepting a bodhisattva as an emanation of some separateness "out there" would be somewhat useless to us, even if we could believe it with the same force that we believe that seeing Jupiter in a telescope means our idea of Jupiter is external and substantial in its own right. You've seen what little good that does in some Christians who accept this literal factuality of God but are hostile to non-Christians. I'll use Chenrezig as an example. The first steppingstone from what I've seen is then the acceptance of Chenrezig as a representation
of our compassion and capacity to love. These are things that even heavily materialistic people can accept, that we can feel compassionate and that we can love.
So we work with that a little while, visualizing, imagining, allowing ourselves to pretend
that there is a deity made of light above us somehow sending love to suffering beings arrayed around us. If it were to remain at this point, it would still be beneficial since materialism can create incredible egocentricity where we don't even entertain the possibility of others' suffering, only our own. It would still be transformative to reconnect with our capacity to call up love at will and practice compassion. Previously we may have thought these were feelings thrust upon us by circumstance and therefore if we felt hateful towards someone, it was just THEM acting on US and that's just how it is. If we felt love, or more often we think this when we don't feel love, it's out of our hands, something done to us by others.
But with this practice, the armor of materialism develops a tiny crack. Maybe there's more at play here than someone "making" me feel a certain way. We called up specific feelings that we may have thought were the effects of external causes. So we pursue the practice further.
When our visualization, our imagined picture of Chenrezig, becomes more and more solid, we can almost feel
the presence of something external to us, like some wonderful friend who has come into the room. Our compassion practice has become so strong that it seems to extend beyond us, and perhaps our materialist minds interpret it as someone "out there" is compassionate. Sticking with the practice, we imagine this compassion emanating from Chenrezig in a physical space near us. You may even have the sensation of a literal being in the room, depressing the floor next to your cushion or only a few inches out of reach above you. But wait, cries the materialist mind, there is no one there, this is all the brain playing tricks on itself. Maybe the presence vanishes from the distraction. Our materialism thinks it has triumphed, but the crack actually grows from this thought instead.
Our minds can call up not only feelings we THOUGHT we had no control over previously, but it can call up the sense of physical presence of another being. Were a scientist to sit there measuring the air around you, they would likely find no one else there, no being occupying physical space. But this strengthens the momentum of what we may be discovering: that mind is the author of reality, that things are not so solid when we stop chanting the mantras of materialism to ourselves. So we continue on.
With the blessings of a teacher and much practice, we may start to have other experiences. We really feel crowds of suffering beings around us, as though we could touch them. The light we envision around Chenrezig begins to actually play through our eyelids, as though closing our eyes in the bright sun. Just observing all of this with detached curiosity, we then imagine Chenrezig dissolving into us; we become someone/something else. We feel we are this embodiment of compassion and love. Ideas of me in here, you out there, are weakened so much as to be utterly irrelevant at the moment. They're on hold. Our reality becomes nothing but droves of endless suffering beings and our compassion for their plight, our ability to help them and their innate ability to help themselves. We may perceive physical signs.
Then we end the session and "me" versus "you" momentarily disappearing didn't cause any great calamity. It likely comes back. Materialists will cry, "It was all in your head. We just saw you sitting there, maybe with a smile on your face. You were hallucinating." But now, having seen the subjectivity of our previous assertions that external factors objectively impact our passive internal ones, that in fact these internal factors can be manipulated such that they give rise to apparently external phenomena with the same believability as a candle on the altar, we begin to doubt the basic premise of materialism: that I am in here, neatly cut off from the external, free to observe objectively as though my mind somehow doesn't exist in the process.
The question arises: What things do I tell myself about reality all the time and accept without question? If I can feel, see, become compassion and Chenrezig, how is what I perceive affected by other such factors that I'm unaware of at the moment? This is a very good thing to investigate thoroughly.
From the relative perspective, we don't work from outside to inside, which in fact is the entire premise of materialism, but rather from questioning the solidity of this inside, of our perceptions and experience, which is often our go-to means of giving legitimacy to the outside we impute with factuality. The Buddha said where there is perception, there is deception.
Here are two problems I see with materialism. The first is when we offer a method of experimentation with Buddhist ideas, they say you must use materialist methods. Materialism already posits "me versus you" as a premise, that "in here" is subjective and unreliable compared to objective and solid "out there" that we can measure. Using any meditation or visualization technique, you'll find this supposition is largely arbitrary. Even for people like your friend who likely won't use "unscientific" methods, some experiments like the double-slit experiment have suggested the very same thing. You cannot slice apart the act of observation into observer and observed as though those things exist inherently outside of the act of observation. They all interact.
It gives rise to important questions. At what point does an external phenomenon become an internal one? Follow the process closely and you will find it harder and harder to pinpoint. So how can you use a method that accepts a framework to be true to test ideas precisely contrary to the framework? It's akin to Christian Science: starting with an assumption (the earth is 4,000 years old) and interpreting evidence to prove that assumption instead of holding it in investigative doubt. I think the problem with materialism is it's self-affirming: this is the ground of reality, so reality is interpreted in ways that appear to confirm the assumption. It's something that few account for in its influence on our experience of reality.
The second problem is that materialism often ignores the impact of internal means of verification, citing their subjectivity, while using exactly those same means to make statements about an external reality. It's as though the scientific method or objectivity were a shelf upon which to set our human mind and body while we work, and that we can be free from ourselves (??) while we measure and make conjectures about reality. It's a very strange and disconnected way to relate to the world, and I think one of the major drivers of increasing depression and anxiety in the US, at least.
So in response to your friend, there really is no quick and easy answer because these aren't quick and easy questions. A way I would try to open your friend's mind to other possibilities though is something like, "When does a phenomenon become observed, what is it observed by, and how does this turn into a fact?" Grill him thoroughly. "I see it with my eye" is not an answer. What part is the seeing? When does the light become a perception? When does the perception become an idea? What makes this idea a fact and not this other one? You may get a response like I've gotten where the question sounds absurd and your friend refuses to answer anymore, but maybe it'll get him thinking at least
Everything is utterly bizarre in its insubstantiality, in its transitoriness. Light and shadows, colors passing through glass, beautiful yet unimaginable and unable to be grasped even for a moment. And still, the play of our minds is astounding in its vividness. Something is lost when we let our curiosity about everything become just another concept to file away with what we had for dinner last night. If you can, don't let your friend miss this richness!