I think the difference between what Ben wrote and your friend's thought experiment, is that in Ben's example there is a conceptual link between the two (i.e. I have experienced this, and can therefore conceptually see how its application could be extended all the way to arahantship), whereas your friend's thought experiment has no conceptual link between the known and the possible, and therefore relies solely on trust.
"Do not force others, including children, by any means whatsoever, to adopt your views, whether by authority, threat, money, propaganda, or even education." - Ven. Thich Nhat Hanh
"The uprooting of identity is seen by the noble ones as pleasurable; but this contradicts what the whole world sees." (Snp 3.12)
"To argue with a person who has renounced the use of reason is like administering medicine to the dead" - Thomas Paine