She should simply be herself without being self-conscious or stressed. I'm not convinced that a person's body shape makes much difference if they have the right kind of personality. There's all kinds of different reasons why people carry extra weight. Maybe the girl might feel inspired to try and lose a little weight by changing her lifestyle choices, because that's probably going to help her feel more confident. Of course, if she doesn't have the kind of personality that clicks with the guy she likes, it probably won't make much difference to him ... but it always feels better to feel healthy so I think she should consider doing it anyway.
I also think she should not be put off by how handsome she thinks the boy is. He might not consider himself handsome at all. He might just consider himself to be Mr Average. If he does think he's really hot and loves himself, he's probably not much fun to be around anyway. In which case, she should concentrate on getting fit and energetic and ready for the next nice guy who comes along some time in the future.