TeenHealthFX is sorry to hear that your friend has anorexia. It is a difficult disease that affects many people, especially teen-aged girls. The good news is your situation sounds promising. Most girls write in to inquire, "Does My Friend Have An Eating Disorder?". It sounds like your friend is aware that there is a problem and is trying to get help. This takes the burden off of your shoulders and actually puts you at an advantage in helping your friend.
Lets talk a little bit about anorexia as a disease, and what your friend is going through. Anorexia is a mental disorder that is used often to help your friend gain some sort of control over her life. She is manipulating food in order to help her feel like she has control over her life. Food is really not the issue; most likely something else is bothering her. For a lot of girls it is low self-esteem or they feel like they do not fit in. In some cases it is much larger issues like sexual abuse or divorce. It could be any number of things and for everyone it is different. The important thing is that your friend is seeing a professional, probably a therapist, that will be able to help her confront what it is that is bothering her and regain the control in her life. Once the eating disorder sets in your whole life changes. Your friend probably feels completely on the outside and different from everyone else, alone. If others have noticed or commented on her appearance, she may even think that everyone is watching or staring at her. No longer will she feel comfortable in her own surroundings.
Here is where you can help and FX thinks that you already have her best interest in mind. Your friend needs a good strong friend who will be there to listen, and support her. If she is not already, she will be going through some very emotionally challenging times. She is going to need someone who will be there through thick and thin. It is also important that you make her feel accepted at school in your social groups, at parties, when you go out etc. Let her know that you want her there. What ever you do do not meddle in her business or ask her about her food. She needs to have complete control over that portion of her life right now. The last thing she wants is someone watching her or questioning her about what she is eating. Try and make life as normal as possible and treat her like you normally would. It is not your responsibility to cure her or even to take care of her, but rather to care about her. If you feel like you are close enough with her, and she feels comfortable talking about it, you may want to ask her about how she is doing. If she knows that you are not going to judge her, she may be able to open up and talk to you about her disease. This is the best medicine. Being a good friend is being a good listener. In all of your helping remember to also take care of yourself. Often times we all get to caught up in caring for others and we forget about caring for ourselves. Good luck to both you and your friend.