Yes, it's a bad habit, but truthfully there are worse ones. I used to bite mine, my mom tried everything. I eventually grew out of it when I was old enough to care what my nails looked like. Maybe you could try painting them for her. Keeping them looking "pretty" might help. But again, we all have some annoying habit, it usually doesn't bother us, just those around us. She's probably a great little girl, focus on the positive things about her! Your attention to the nail biting may be making it worse!