I disagree that "Saying you don't like someone [in a sexual way] anymore because they're fat is mean". It's just a statement of fact, I think; it's not like he said he doesn't like the girl as a person because she gained weight, just that he lost a sense of sexual attraction to her. It might be shallow, but it's certainly not immoral to say that a change in someone's physical appearance has changed your physical attraction to them.
HOWEVER, I think I agree with you that showing the photo wasn't a good move, and stepped over the line.