Some kids just need lots of sensation to self-soothe, to deal with boredom or anxiety or anger or fear, to compensate for lack of activity, for all sorts of reasons and non-reasons. For most of us it's a sensory integration issue – there is a real, though indefinable, need for some stimulus. Nail or cuticle biting is one of the more common ways people meet that need.
I bit my nails from early childhood through my senior year in high school, and for several of those years, I struggled to overcome the habit myself. I even used my allowance to buy my own nasty stuff to put on my nails. It is a very compelling habit.
I finally succeeded through a combined approach: I kept noticing when I was doing it and why, if there seemed to be a reason, so I could reduce those situations as much as possible. I carried a file with me to smooth my nails, since any tiny rough spot could trigger a relentless need that I could actually FEEL in my teeth. I nibbled hair, chewed erasers, nipped at anything within reach. I also wore a pendant that felt good to touch, or kept a smooth stone in my pocket, or substituted other habits like twirling my hair, stroking clothing textures my fingers liked, and even folding a piece of scratch paper and gouging fingernail dents all the way around. I still like that one, and I have to have fingernails to do it.
I hope you won't nag your daughter (or hubby) about the habit. Nagging might only increase the need to chew. It's not terribly hiegenic, but most nail-biters don't die of terrible diseases. Let her know you hope she'll stop biting because she will really want to have pretty fingernails some day soon. (But please don't put toxic nail-polish on her nails – you can use a buffer to keep them shiny and lovely.) Ask her why she wants to bite them, and why she might want to stop biting them. Ask her if you can help her find other things that would feel even better. Let her be part of the solution.
