She could have also gone through some kind of serious trauma & the stress of it might have causes the extreme weight loss....it can happen even to the point of being hospitalized for anorexia (extreme weight loss) like happened with me. I was always at the lower end of my safe weight zone for my height & stress always made me loose weight....it makes me feel sick so I have a very difficult time eating.....going through a long term trauma I ended up in the medical hospital needing IV nutrition through a central line.

There are many reasons for extreme weight loss including illnesses & sometimes people/ even family don't want to let others know about it.

Not all people who have anorexia have "anorexia nervosa" which is usually based on body image issues...but are usually start from childhood issues or bad situations that have hit them in life.....& yes, I know that even my MIL got started on a diet & it worked so well, she lost way too much weight...couldn't stop loosing the weight because she had so changed her eating habits