Hi D.,
Poor girl! And poor YOU! I am a nighttime teeth grinder myself, and have been since I was very young (my parents noticed it as soon as my teeth came in) so I know from first-hand experience how awful this can be for both the grinder and the one listening to it! Anyway, I now have a nighttime tooth guard made from the dentist especially for my mouth. However, they do sell ones that you can mold to your mouth at Walgreens for around $10 I think. I forget the name brand, but they are NOT the ones for athletes, they are specifically designed for nighttime teeth grinding and they fit differently than athletic guards. Contrary to popular belief, it's not necessarily stress that's causing the grinding, so yoga probably won't help. At least in my case, the only source of relief (for my teeth and for my husband's ears) is a tooth guard. If I were you, I would definitely get her the guard they sell at Walgreens for her stay with you, and I would also talk to her parents about getting her a more permanent solution. In most cases it's a life-long battle, and can cause serious dental problems, migraines, neck stiffness, jaw problems, etc. etc. Good luck!