I was a teenager that had an eating disorder. I was also a young adult that had an eating disorder, when I felt like my life was out of control. the lord cured me of a 10 yr battle with anorexia/bulimia.
I can tell you one thing, the more you focus on her eating, the more she will do the things that bother you and people in general. If you focus on her not eating, she will start to eat and just go throw up. There is no reasoning with her that will get her to see that she is not fat. nor is it really about that.
what would be better is to get her into counseling, and find out what makes her happy. that doesn't mean you don't correct her, walk on eggshells or try to accommodate her. the hospital told my mom to make certain types of meals with measurements. Never lasted. my mom would do it, and I would eventuall stop eating.
If you feel that she is in danger of herself in any way, either put her in a hospital where they can put a tube in her, or put her in a long term treatment center so they can get her help. let me reiterate that there is nothing you can do if this has turned into an eating disorder, besides treat the symptoms. You can't make her eat, and the more you try, the worse she will get. if she was overweight, it would be the same way. You can harp, and she will just get thinner.
Now, all this to say, is that you need to love her and show her you care, and you need to use tough love if it gets to that. do some research. Look at the grocery aisle, you will see what kind of pressures are put on our girls.
just don't make a big deal out of it. do the counseling, and get her medical help if she needs it, matter of factly. You may warn her, but it may not do any good. giving her nutrition information will just help her get around eating normally. she will figure out what has the least calories and all that.
do you know what really cured me? My overbearing family getting out of my business. I did better when I wasn't in the family. they were overbearing and i was showing my independence. And, pray. Now, realistically you have to take care of her, and so it can be really hard, to want to make it better, and not being able to as the parent. you will just have to let her know that she has to do xyz; counseling, hospital, psychiatrist, etc, unless she eats right and stays at a healthy weight. stay firm on those things, but do not concentrate on what she is or isn't eating.
And, no, she probably won't just snap out of it. you may want to tell her the side effects of anorexia; like health issues current and future (hypoglycemia, low metabolism, etc as an adult), and possible infertility, heart problems, etc. if it is a phase, it may help, if it has already turned into eating disorder, it probably won't.
i hope this helps. what cured me temporarily was pregnancy once an adult, because i am really pro-life, but after a few months, i would fall back into it. until the Lord released me from it. Good news is that it isn't that common in women over 25yo.