Two months into a monthly allowance, my 15 year old has become the bank for her friends

Started in April as an experiment. Fixed amount, she manages it, I don't ask. She's managed it well. The problem is two friends who borrow 'till next week'. One has paid back. The other owes her more than half a month's money, and my daughter won't ask because it'll be awkward. I'm trying not to step in. But I also don't want her learning that the polite thing is to quietly lose the money. How far would you go?