Your girlfriend should see her doctor and then a psychologist who specializes in eating disorders. She may or may not have anorexia and a doctor should determine that. Encourage your girlfriend to see someone.
Many teenage girls are concerned about their weight because of societal pressure and sometimes don’t eat enough food. Anorexia is characterized by a preoccupation with thinness, excessive exercising, restricting food intake and a distorted body image. The reason she is not eating is probably not food rather a psychological reason.
Her healthy body weight is 96 to 117 pounds. If she is a vegetarian, what does she eat? See the vegetarian topic for more information about foods and meal patterns. Soybeans are a complete protein and would be good to include in a vegetarian eating plan. Green foods are usually from a vegetable source and any would be fine to eat but are low in calories.
The best thing you can do is being positive and supportive. Your feedback could include saying is what you feel she is doing – not eating – is unhealthy. Any nagging you do about her food intake or body will probably make her more resistant.