basically, I've found no way to reason someone out of gambling on crypto when they really want to. The main thing is to try to make sure they don't gamble away much more money than they can afford. But people doing this don't listen.

At best tell him to wait for the price to go down. Then when it goes up, get him to sell his cost basis (what he originally paid in cash) so that everything from there is profit.

This will not be easy.