You are not the definitive authority on what is beautiful. Plenty of people, myself included, find her beautiful (maybe have a look at her website and not just this photo)
Insulting her is rude. Telling her to go eat a hamburger is rude.
If she had an eating disorder, she is not bad, weak or stupid, she has a mental condition.
This would be like saying to someone in a wheelchair to just “get up and walk lazy”. It’s not that simple.
Also if you want to gain weight you should be eating healthy nutritious foods, not hamburgers.