I am really sorry that you are experiencing this.  It's never nice to deal with something like this.  However, I am very proud of you for seeking help about it.  Remember that you're never alone.  There will always be someone, including myself, who really does care and want to help you in any way that you can.

It's very unfortunate that your friend feels this way.  Unfortunately, you cannot force her to seek help.  Helping someone who doesn't want to be helped is completely pointless.  In order for her to get better, she needs to want to help herself.  There's no other way for her to truly get over this.

Here is an excellent link that is geared towards eating disorders.  I hope you find it helpful:

Information on Eating Disorders

Have you tried talking to someone about this?  Talking to your parents about this can be really beneficial.  They have more knowledge than you do and therefore, may have their own ideas and advice on how to deal with this situation.  Maybe they can talk to your friend's parents in private for you.  I understand that you don't want to get your friend in trouble but an eating disorder is very serious and you could potentially be saving her life.

If you really don't feel comfortable talking to your parents about it, why not talk to a close friend or family member about the situation?  If you don't feel comfortable talking to any of the people I've mentioned above, you can always go to a guidance counselor.  Guidance counselors specialise in situations like your own and therefore can provide you with quality, professional support that can help a lot.

Whatever you do, you *need* to tell someone.  I think it would be ideal if you talked to an adult about the situation.  Like I said, it is evident that you really value your friendship and don't want to break the trust of your friend.  However, you need to do something immediately before things get a lot worse.  With eating disorders, things can severely escalate very quickly.  It's really scary.  It's not fair that you have to deal with this all on your own.  It's too much for you to handle and therefore, I strongly recommend that you seek help immediately.

I hope I helped.  If you ever need anything or feel the need to discuss this further, feel free to message me any time as my inbox is always open.  I am always happy to help.

Good luck and I hope everything works out with your friend.

DYL

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You're like a bullet to my brain, I'm numb, can't kill my fun.
If you're good and done then thank you just the same.
I don't owe you anything.

I would like to second what DYL said, you need to tell someone about this. You alone cannot save your friend, and no matter what you have promised, it is in her best interest for you to let adults, and other safe people know about what is going on. She needs help and it is not your job to save her.

Be there for her, encourage her, support her, love her, but allow professionals to contribute to the conversation as well by helping her get the help she needs.

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Do not abandon yourselves to despair.
We are the Easter people and hallelujah is our song.

Pope John Paul II