I'm trying my best to think of something that you can personally do to help prevent her from falling down this slippery slope she's treading. I'm really trying my best to remember what it was like when I personally first started exhibiting those same behaviors, and what I feel would actually be helpful if someone were to step in and say something to me. First of all I feel like most people aren't properly educated about anorexia. So they don't necessarily know the effects of anorexia, on what it does to both your body and mind, and what happens to you after recovery as well. So I think it might be safe to say, she doesn't necessarily know what she's getting herself into.
I would probably say to myself. It is NOT WORTH IT. No matter how fat, or disgusting, or ugly you may feel. Anorexia is probably among one of the most horrible disorders out there, because it controls virtually every aspect of your life. It completely consumes who you are and your personality. All you will think about is food constantly. Because it is what we humans need for survival, and anorexia goes completely against every natural instinct we as humans have to survive. All you will think about is food, starving yourself, and wanting to be a smaller weight. IT WILL NEVER BE ENOUGH NO MATTER HOW MANY POUNDS YOU LOSE. You will always want more. You will always feel fat. You will never feel happy. It is a horrible, terrible way to live, that slowly kills both your body and your soul. It will literally shrink your brain. And what's funny is you will have to keep starving yourself forever to remain that skinny. If you truly truly recover, and start eating like a normal person again, the effects of anorexia slow down your metabolism so much, it's very likely you will regain all that weight back. It is not a solution. It is not worth it. It will not make you feel any better about yourself or your body. You WILL feel a MILLION times worse. And it will be all you can ever think about.
I suggest maybe providing a healthy alternative, if she truly feels she is fat and truly feels like she can't be happy with herself/love herself and her body at her current weight. Try suggesting an alternative form of dieting, that is effective, healthy, and doesn't have long term damages mentally and physically like anorexia does. Maybe suggest keto, or cico. If you aren't aware of those diets, I suggest you research and look into it because it actually does help and work for people. I can't stress enough that the goal should never be to be skinny, it should be to feel HEALTHY, HAPPY, and CONFIDENT. And you need to reiterate, that the goal first and foremost should always to be healthy, and to achieve that in HEALTHY WAYS. Maybe you can suggest going to the gym with her as well? And also do not tell her that she needs to diet, just the next time she mentions to you she's starving herself be like, "Starving yourself is extremely unhealthy, you may lose weight initially but it doesn't work long term, and doesn't last because the second you start eating normally again you will regain all the weight due to messing up your metabolism. How about trying KETO/CICO/VEGAN diet/exercise/etc. instead, it will help you lose weight in a HEALTHY and SUSTAINABLE way." Make sure you let her know you don't think she needs it at all but you are only suggesting this because you want her to feel happy and healthy in her own skin, and you want her to do it in a HEALTHY way, and not in a way that can be so damaging and destructive. And remind her that you will always be there to support her. And that she can always rely on you and that she can go to you if she feels unhappy or insecure, and you truly do have her best interest at heart.