Hahaha, she's fat. The horror. The betrayal. He thought he had a girl who loved him, but little did he know she was fat. And nothing is more horrific than that. Thank God he found out, he might have started caring about her, like an idiot.
When he attempted to reverse search the image, he didn't find any matches which lead him to believe this is a real photo from the person he's chatting with and not a joke or something like that. There are two assumptions (based on this thread anyway, order has no bearings on which one is more "true" or not).
(My assumption) He is disappointed because the girl is fat/hairy and therefore unattractive and unworthy of his affection.
He is disappointed because the hairy leg makes him think his "girlfriend" is his "boyfriend," otherwise known as a catfish, and he is feeling tricked/embarrassed/hurt/etc.
My knee jerk assumption was weight. When I encounter posts with fat people, I'm used to people responding in revulsion so I took that train of thought.