good luck robin, I chickened out of talking to the boy directly after a few attempts - I did say the odd thing, like 'would you mind sharing your sweets' but didn't really get down to the nitty gritty of give and take as I saw it. Perhaps I should have, but I wasn't sure what his mother would have said if he'd repeated my homily back to her.
FWIW, I think it's a good idea to give your dd her money back this time, and tell her that not all people who borrow money will pay it back, so she must remember that in future. Hope this girl and your dd stay friends, minus any bad feeling. But not always possible, I know.