Her unwillingness to eat may be a sign of an eating disorder or it may be because she actually feels like she can't eat. Either way, she should get medical attention soon, because if she's lost a noticeable amount of weight and doesn't want to talk about it, there's probably something to be concerned about. The best way to help your friend for the time being is just to be there for her. Try to get her to tell you about her weight loss and look out for other signs that she may have an eating disorder. If you find out that she does, then try to be supportive and encourage her to try and stop. The longer that a person has an eating disorder, the harder it gets for them to get over it, so if she does have one then time is of the essence.
It also wouldn't hurt for you to tell an adult about what you've noticed. Your parents, her parents, a teacher or counselor; someone who you can trust and who cares about your friend. Even if she doesn't have an eating disorder, not eating isn't healthy, and she may need medical attention. Does she seem depressed or stressed? All of this may be because she's got a lot on her mind, or maybe she's anxious about something. If it started three months ago, it may be because she hasn't gotten over her boyfriend yet. In any case, taking her to see a psychiatrist could help her to get over whatever it is that's made her stop eating.
More than anything, like I said before, just be there for your friend and listen to what she has to say. She may resent you if she has a disorder or ignore your help if she's depressed, but if you end up helping her then chances are she'll be grateful. Maybe get some of your other friends or even her ex involved and you can all talk to her about it and see what's going on. Make sure that she knows how much you care, and that no one wants to see her suffer in any way. She may just need someone to help her.
Good luck, and if you need anything else feel free to message me.
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Sexual innuendo.