I think that it is fair to ask her piano teacher or whoever (assuming it's not you) to explicitly talk to her about how to practice. Not just "you need to practice 15 minutes a day," but "When you practice, I want you to first spend a minute breathing and relaxing your neck and shoulders. Then you should play scales, trying to make them as even in timing as possible, for 5 repetitions. Then, choose a piece of music, and decide which part of it you are going to work on - the dynamics, or the notes, or the rhythm..." (I guess I would say the same thing to you if you are the one teaching her to practice, but you could do that during practice time instead of lesson time if it is. Just make it clear that you're teaching how to practice now, not teaching the music itself.)
