If she is overweight
and thinks that not eating is going to help her lose weight than she is wrong. When you do not eat it slows down your metabolism
(in other words your ability to burn sugars & fats). The best thing you can do is suggest a healthy diet( i.e. three meals a day with protein and lost of fruits and veggies- avoid food with refine sugar and high fats, or anything that states hydrogenated) and an exercise plan, maybe three a week to start off with (cardio & lifting weights are important to build muscle which burns fat).
If it turns out to be an eating disorder
, coming from experience, it is really hard for you to do much besides being supportive and encouraging/trying to get help for her. Eating disorders are often based on psychological issues therefore when she sees herself her body image is possibly bigger or distorted to her than it actually is. She can't see what other people see, therefore professional help is probably the best thing for her. I hope this helped a little bit and I hope your friend is okay.