Why would she need to eat more? People are weird. Even trophy girls these days would have been called chunky only a couple of decades ago. I repeat.. people are weird.
Girl I dated once was 19 and when she raised both arms at once I could have played her ribs like a xylophone and she was 100% healthy. Long, lean and sexy. One of the smartest most capable people I've ever spoken to as well. Unless you're her doctor or her doting mother, stop telling thin and healthy girls to get fat.
By the way, she's awesome.