aaargh - poor you, poor her. I used to do this. It's a horrible habit. I find plasters are the only way forward, I've never found anything else. But you have to get the right plaster that will stay on for a full day and be able to bear lots of picking at, which is what will happen when she can't bite them. I think fabric plasters work best. Oh yes, the other thing worth trying is putting some calendula ointment on, it is a healing ointment which obviously helps and is also very slithery so harder to bite, Doesn't taste great either.
However, is very likely there is something more to this. Is she stressed? Worried? nervous about something? It might be worth a trip to GP if you have a good one who will be able to talk around the issues and try to determine what is really going on with her.
FWIW, I seem to have stopped in the past three months, since I have started regular meditating and am much calmer.