Is she biting them compulsively until her fingertips are unhealthy, or just trimming her nails with her teeth?
I only bite my nails to trim them and have since childhood; if I see the tips of my nails turn color and get that rim of extra nail, I look at it like, "oooohh they look tasty" and can't not want to cut them off with my teeth.
I remember once my brother sleeping and his nails were long, and I saw his hand (we shared a room and our beds were next to each other) and I was fighting my impulse to bite his nails LOL.
"Ah, those endless forests, and their horror-haunted gloom! For what eternities have I wandered through them, a timid, hunted creature, starting at the least sound, frightened of my own shadow, keyed-up, ever alert and vigilant, ready on the instant to dash away in mad flight for my life. For I was the prey of all manner of fierce life that dwelt in the forest, and it was in ecstasies of fear that I fled before the hunting monsters."
Jack london, "Before Adam"