It's very common for folks who suffer bipolar to do that sort of thing during their manic cycle. Whether or not, strictly speaking, he can "help it" is a question I can't answer so I'll let other address it. In any case, I think it's perfectly reasonable to get the money for rent when his check first comes in rather than waiting 'til it's due. If the friend understands that he's likely to spend it all on the "upswing", hopefully he'll be cool with setting aside the rent before that happens.