My daughter often lends money to friends who never seem to pay her back. How can I help her be more careful?
Kids have their own social structure, which is characterized primarily by not having anything to do with the adult world. One of its secondary characteristics is that kids will borrow money from one another and lend money to one another. You're not going to change this.
You can talk about how important being clear is when lending money instead of giving a gift. Even if it's embarrassing, your teen has every right to ask a few questions and get a few answers: How much does her friend want? Why does he or she need the money, and when will she be paid back? If she can't get answers or she doesn't like the answers she gets, help her understand that she doesn't have to say yes. It's an important part of empowerment to be able to say no to a friend for whatever reason.
Talk with your daughter about these issues. Talk her through some hypothetical situations. Do some role playing. This is one of the most difficult situations your child will face, and role playing can help tremendously to rehearse her for facing these situations if and when they happen.