First glance, this sounds like an embouchre problem, perhaps. Though the bottom lip is between the teeth and mpc, it's actually supposed to be more BEHIND the bottom lip - not directly under it. It requires building up the lower lip muscles to manage this - exercises in Larry Teal's "Art of Saxophone" book help somewhat. This can help tone control as well. Some people have dental inserts made which a player places on top of their bottom teeth (only when playing - I'm not talking surgery) to "round the edge" of the lower teeth. This is really a band-aid of sorts - it's better to do things right, though I can se how a person that plays for many hours may develop numbness even with a good embouchre. Everyone's physiology is different, so the solution for your daughter may not be the exact solution that works for me. She should consult with a professor on this very soon.