The best advice would be to pay it if you want to, offer her the money to pay it, or just wait for her to make the next move.
If you lend anyone money, only do it if you can afford not to get it back. I never "expect" to get it back, especially if it's for family or very close friends. If it comes back, fine; if not, fine. I don't want hard feelings over money. And yes, the Bible says "don't be a lender or a borrower", which I take to mean give the money without expecting to get it back.
You are a good friend for letting her know about it before she gets home.