Loaning money to friends is a great way to ruin a friendship. As CB and others have stated, better to just give them a gift, and be pleasantly surprised if you get paid back. (I'm batting about .500 both inside and outside the hobby in terms of being pleasantly surprised this way.)
Loaning money to acquaintances seems like a terrible idea. Here's why:
When considering an unsecured loan, I ask myself two simple questions: Do I trust the borrower? And do I think the borrower will have the means to pay back the loan? With most acquaintances (and hobby relationships almost always fall into this group) the first question is a crap shoot. And the second question is problematic - if someone who charges $200-500/hr is so broke that they need to borrow the equivalent of less than a day's pay, is it likely they'll start managing their money well enough to pay back the loan?
There are exceptions to this rule, of course. But they're not easy to spot. If you want to play bank, better to just give the money away.
(Of course, when a provider is asking for a loan, "How about a session, so you can earn it" is the obvious question. Very rarely will the answer instill confidence. Again, there are exceptions.)