If we're talking about physical looks alone, then I'd say he's too good for her. He's fit and attractive and she's chubby. She has a nice face but chubby.
The reasons why he doesn't have a chance is because I can't see her being interested in him. And he's oblivious to it. He's a high school senior. He's 18. She's 23 and in college. I feel like he might just like her because she has big boobs but he insists he likes her for her personality.
But how can I make him understand that a 23 year old isn't going to be into an 18 year old guy?