Dog chews off her toe nails?

My moms dog chews off her nails to the quick. Then is in pain because she chewed it all off. She came from puppymill rescue along with her sister. She eats holistic, is always with her playmate, gets lots of attention. She recently began doing this. I am wondering if there is a disorder, or breeding malfunction, or habit. My own dog loves to eat the nails I have clipped, and even eats the clippings from horses. I understand they like what ever minerals/vitamins nails give, and it is healthy for them to eat nail clippings. So I wonder if something is missing in her diet to cause her to chew off her own nails? What is in nails that mom could give as a supplement?