Weight loss and tooth decay?

Hello! My mother has Lupus, and had a gastric byp last March. She lost more than 200 pounds. But problems have arisen: First, with her teeth. They are decaying, cracking, and forming brown spots in between them. She is not a smoker or a drinker, and has never had problems with her teeth before. She also has gone underweight and is now very pale with bags under her eyes. And now she also has problems with fluctuant swelling in her legs. Is there any guesses as to what may be wrong, and how to help stop it? (Sorry for the bad English.)