Girls at 5'5 are suppose to be anywhere from 110 to 140. I know a few who are around 110-120 and look pretty healthy. The main thing is that she doesn't continue to lose weight and especially if she loses a lot of it fast. Try to be there for her and get her to eat high calorie foods..you'll have to sort of trick her somehow by making it fun so she won't argue and all that. People don't want to eat when they are being chalenged or are depressed which is true in her case.
Even if you get her to eat few things that are high in calories, at least it will keep her going. I don't know what age she is but as an example at 25, 110 lbs being 5'5 you need around 1500-1600 cals/day which is fairly easy to get. Good luck!