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If she's not anxious or nervous, her nail biting is essentially a combination of boredom/lack of interest in the present, unless she's doing it compulsively in a way that would suggest obsessive compulsive disorder or something similar. I don't think there's a real "fix" to get rid of her nail biting habit; perhaps let her get fancy nails with nail polish and she won't want to destroy them. :)
Mean and also pretty ineffective (she could just go wash her hands). Perhaps rechannel the behavior into another activity, like redirecting her to do something constructive when you notice her biting.
Good idea :). I hope my suggestions were able to help. Thanks for using JustAnswer!