I can relate to your daughter's woes. I started biting my nails when I was 2...and then I started biting my toe nails when I was 3...no kidding. I'd drop down in the middle of the sidewalk in front of our house and pull off my shoes and start tearing into them.
Thankfully I don't do that anymore, but I still obsess over my toe nails and pedicures and I bite the daylights out of my nails. It's a real bummer. I've tried manicures and fake nails, both gels and acrylics. I've tried the bitter-apple nail polish that tends to deter most 'normal' people from biting--again, I don't care about the taste, I just care about the nail. It's a terrible habit, but my mother taught me that if I'm going to do this I better wash my hands because of all the stuff I touch throughout the day goes into my mouth. And so I wash my hands a lot, but not obsessively. But she also scolded me a lot about my nervous habits, which in turn made me an even more nervous and shy child.
I do think though, if my mother took me out to have manicures every other week I'd probably have a whole new appreciation for my nails and self-care. The temptation to bite them might have been swayed into the temptation to keep them neat and pretty. I love neatly manicured nails still today and envy women and men who have them. So, I recommend from 30+ years long of nail biting that you continue to support the good behavior with rewards like nail polishes and manicures and if she bites them, just let her know she'll need to wait until her next manicure to have them fixed.
Good luck!
I.