I'll state the obvious, if no one else will. And I am not picking fights, just observing. Your friend is a Mahayana Buddhist. They run the risk of burning in a Hell realm or accruing "really bad karma" if they take a precept and fail to uphold it. Likewise if one starts out on the 8fold path, but then quits midway, "really bad karma" results, and disastrous results are expected. If one holds such a belief, it makes sense to practice a lot more caution before proclaiming oneself to be a Buddhist, and one needs to be especially careful around newcomers.
If you do not fear the supernatural repercussions of making mistakes or acting out ignorance, then taking the precepts for a day or a week is better than nothing at all. Try and try again, to whatever your wisdom is developed and to whatever extent you are still bound by ignorance and delusion. You are fighting a circular argument with your friend; respect that his position is oriented in a dramatically different POV.
"As I am, so are others;
as others are, so am I."
Having thus identified self and others,
harm no one nor have them harmed.
Sutta Nipāta 3.710