Alright, here's my advice. I know I'm new, but I think I have some wisdom to give, as I live with obesity. For those not as word savvy, this means I am overweight. Ask her if she has seen a height-weight chart. This chart tells you the weight you should be around for your current height. This helps to show signs of becoming too heavy or too light. If you eat too much, you get fatter. If you eat too little, you get lighter. Is she exercising very little? Too much? Just enough? Get her to check out one of these charts. Most doctors should have access to one. Seeing this, she can check what her weight should be at. Then, ask her if she's comfortable with sacrificing her health for her looks. Because a lot of people that give up health for beauty can die. That's no way to go, I wouldn't wish it on anyone. Just tell her that one's looks don't matter, especially not in a relationship. They can reel the guy in, but it's the other qualities that land him. As a guy, I can tell you, I like a girl by personality and smarts. Looks are third place, as they aren't so important. It's one thing to look at a girl and be attracted. It's another to KNOW her and be attracted.
If she's doing this to get guys to notice her, it's the wrong way. You gotta be healthy first and a looker second. If she opts to become healthy and emphasize other qualities, like her personality, how smart she is, and how she acts, she can land any guy. Looks aren't everything if the whole package is lacking.