KDC the problem he wil have is that the hardener for car paint is super expensive and probably they wont sell him a pint or maybe a quarter of it, that thing at least here is for the gallon and buy a 150 bucks hardener is not cheap at all, specially if you do not paint cars. Besides if he mix the hardener, the paint and the thinner wrong it means to ask for troubles.

You can paint using car paint w/o using hardener, but you need the thinner and a good clear coat. You can buy thinner in quaters just in case and is relatively cheap.

God luck OP