My 14 year old DD bites her nails. She has been trying to grow them, as she is desperate to have them looking better for our upcoming half term holiday. They aren't a very good shape - pink bits are quite wide and short and white tips look too long in comparison.
Long-term nail biter here, only very recently kicked the habit. My message to your daughter:
I coated my nails with Sally Hansen Maximum Growth. Sally Hansen products are very hard wearing and will help keep your nails protected (both from environmental stress and your nibbling) without chipping. Personally I lose the temptation to bite when the texture of my nails is all weird. The trick is to keep applying Sally Hansen until the temptation to bite is gone. Your nails will grow out long before this happens, so make sure you don't relapse!
You'll probably find the white bits on your nails get tons of dirt under them. This only happens in the early stages as your nail tips are still almost flush against your skin. Don't dig the dirt out with tools or your other nails as this will separate the nail from the bed and shorten the pink bits even further. Use a soft nailbrush to clean underneath and if you do need to apply more force, be as gentle as you can.
Your nails will still be a bit uneven for your holiday, but if you can grow the white bits then a salon can work with that. Even at this early stage, a basic filing manicure will massively improve the look of your nails. I wouldn't recommend acrylics as they may damage your nails and put you off keeping them nice. Gels can be damaging too but nowhere near as bad as acrylics. Your best bet at this early stage is probably just regular nail polish.
It's a huge challenge to try and stop biting your nails - non-nail biters have no idea how difficult it is. But even if you relapse, don't be too upset with yourself. You've proven that you have the self-control to stop! You've got this!
Also OP, I would definitely recommend using the salon as a reward (even if you don't take her very often!) I've been getting the occasional manicure to treat myself for kicking that 20-year habit and each new manicure makes me feel very rewarded and much less tempted to bite.