If a woman suffers from anorexia for over a year, how severe might internal damage be?

Varies. It really depends on how much weight loss and over how long of a period, how much it did affect her electrolytes (blood salts), hormones and other parameters. The rule is, the sooner the treatment the better the response and resumption of normal functioning.
Anorexia. Depends on how mal-nourished the person is and whether or not treatment has been successful. This is a very dangerous disorder and you should be consulting frequently with your physician to help get you well. Never give up. Try to follow your doctor's advice.