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.