Hi! I bit my nails very badly until about the age of 12 or 13. I tried to stop but just couldn't. There was a product then called "Nail Biter", which was HORRIBLY bitter. It really did help, but didn't really cure the problem. I have had good luck with Orly products, and they make something like that. You can find it here: http://orlybeauty.com/treatment.php?c=1&gclid=CKaq7db...
Also, I know this probably sounds crazy, but I used to glue nail tips on. Even though I always had them in my mouth anyway, I think it might have helped. I don't generally advocate acrylic or gel tips, but a nice manicure (not necessarily tips) with a gel overlay (to thicken & harden the nails) will make them look nice and maybe she won't want to crack them by biting. An overlay will also help nail polish to stick, and I did that for a long time just to make my nails thicker and prettier. If they look nice, maybe that will help too. :) That said, if you do take her to a salon, go to someone reputable and pay real $ for it. You don't want to take any chances with her health. Ask them their cleaning procedure and insist on clean tools and that single-use disposable emery boards and things - be new and NOT used ones. If they don't have a good answer for you on the sanitation, don't feel rude just simply walking out. If you want to do it at home, get some Orly Bonder. http://www.orlybeauty.com/treatment.php?c=57 This stuff is AWESOME, rubbery basecoat that will help nail polish to stick much, much longer, even with play, water, and everything. It's just easier not to bite your nails when they look nice. :)
Give her a good verbal "tour" of all the thing she touches each day and explain that all that dirt & bacteria is on her fingers too. Maybe it'll gross her out of it! lol.
I still chew on my nails, but I don't break them with my teeth except by accident. I always carry an emery board with me (on my keychain, in my purse and in my car) so that when I get a little snag or uneven place I can file it smooth. When your nails are smooth, there's less to mess with & chew on. My nails are long and pretty now, but they are still thin, so keeping them filed and smooth really helps the biting. Over time, my nail beds have also gotten longer, so my nails look longer even when they are short. It takes a long time, but they will start looking good.
It's a very difficult habit to stop, but if she really wants to stop, please let her know that she CAN DO IT!
I hope this helps!!
S.