I bit my nails from the time I had teeth (well, from a very young age ;-) until adulthood. My mom had tried many things, but my husband finally helped me stop. I told him how much I wanted to stop, and he made me agree that I would do whatever it took. Oh, I regretted that agreement for some time, until I was finally nail-biting free. Because any time I put my hands in my mouth, he would say something and/or reach over and push my hands away from my mouth. It's a miracle we stayed married!! It took a year or more, but I stopped. I'm not necessarily recommending this exact approach, but just pointing it out every time would help. It will get annoying to her, but I think she'll be glad in the end. It becomes almost involuntary. She probably doesn't even realized she's doing it 90% of the time. But not just you...your her mom. She needs to rally the support of those she's around most. I completely agree with someone else's advice to get her friends involved. They should point it out when they see her do it. Maybe if she has really good friends, and they have a REALLY good relationship, she could even have them push her hands away. I'm telling you...that drove me so nuts that I went from biting, to hiding the biting, to stopping all-together...just so I wouldn't have someone pushing me around. Hahahaha!!!
Another thing that helped VERY much was to keep nail clippers and a file on me AT ALL TIMES. Maybe she can let her teachers know about it & ask permission to pull them out during class (so they don't think she's just a primper who is not paying attention). Because I could not stand to have a nail that was crooked or snagged or anything & would bite it at the first sign. But if I had my "tools" on me, I could whip them out & groom, rather than bite. And I bought myself a pretty natural-looking nail polish/strengthener that I would paint on morning & night. It helped strengthen my nails, I would try harder to keep the polish job looking nice, and it didn't taste so great!
And one last thing. I don't know that a kid would really appreciate this aspect enough for it to help, but she should do some research or something on the amount of nasty stuff on our hands, and under our nails. Ugh...now that I no longer bite my nails, the thought of putting my nasty hands in my mouth really grosses me out (and my hands are way cleaner now than back then!). But I remember people telling me that when I was younger & it did nothing to stop me from nail biting. But it's worth a shot. And maybe if she does some of the research on it herself, it will mean more to her.
I know how she feels. It has a lot to do with a lack of self-confidence and/or stress. I'm still a high-strung person, though, but one who no longer sticks her fingers in her mouth. So it can be done. But she really needs to make a commitment to stop. Maybe you guys could make up a little certificate where she promises to give her best effort to the cause. Attach a reward to it. Maybe you could take her for a manicure when she has been bite-free for a month. And maybe buy her a pretty silver ring once she has stopped biting for 3 or 6 months.
Okay...I'll quit rambling. It brought back a lot of memories! Good luck!!!!!
C.