The best thing you can do is not address the problem straight forward (at this point-- she will feel threatened and might leave you as a friend). I would suggest (having been on your friend's end) say things like "I want to be as skinny as you are, the only way I could is if I freakin' starved myself" she won't want you to do that. Another way is if she starts getting bruises and things on her body (she's gone pretty far at this point) you can yell at her and say it's because she doesn't eat.

Some signs:
Picking at food/avoiding food
Taking food to another room to eat away from everyone
Bags under her eyes
Weight is significantly gone from her inner thighs/back of arms/under chin
Bowel issues
Her period stops
She lacks energy

This could be a serious issue for her heart, eventually after her stomach has depleted her fat stores, it will go after the muscle. Her body will weaken even more. If that happens, you will not be able to get through to her, you will have to talk with her parents, or some one that has a sort of control over her.

An ED is based on her controlling herself. You might want to try to go into the problem by fixing whatever she is trying to control that is going wrong in her life.

Her loss of weight could be a number of things. Has her personality changed at all? Does she have a significant amount of stress in her life? If she seems to have an issue with food itself, then she probably has an eating disorder. However, if it's just a loss of appetite, or she seems to be losing weight without cause, it could be depression, disease, or she over-exercises. I would keep an eye on her, scrutinize (without making it obvious) the cause of her weight loss, then take action.

Exercising too much is also included in the ED category-- if she is actually suffering from and ED, she more than likely hasn't quite realized what is going on completely. If she is "pro-ana/mia" she is purposely doing this to herself and not only created and ED but has a mental issue as well.

I've noticed that people who have eating disorders often have had some sort of trauma in their past or are having a bit of trouble controling things in their lives.

Eating Disorders are characterized by a disturbance in eating behavior. There are two main types of eating disorders, Anorexia Nervosa and Bulimia Nervosa. Anorexia Nervosa is characterized by a refusal eat that leads to a below normal body weight. Bulimia Nervosa is characterized by repeated binge eating followed by self-inducing vomiting, misusing laxatives, fasting, or excessive exercise.

Anorexia Nervosa
Most often diagnosed in females (up to 90%), Anorexia is characterized by failure to maintain body weight of at least 85% of what is expected, fear of losing control over your weight or of becoming ‘fat.’ There is typically a distorted body image, where the individual sees themselves as overweight despite overwhelming evidence to the contrary.

Bulimia Nervosa
The key characteristics of this disorder include bingeing (the intake of large quantities of food) and purging (elimination of the food through artificial means such as forced vomiting, excessive use of laxatives, periods of fasting, or excessive exercise).