While what Acchan actually said was rather harsh, I have no doubt that Acchan is a good friend. He's certainly clumsy in his attempts to 'help' Kouta, but his intentions are good. He wasn't saying it because he wanted to put Kouta in his place or make him feel bad about his looks. I think he's just concerned that a 'weirdo upperclassmen' might be taking advantage of his friend. I think what he was trying to say was more "think this through! this guy doesn't make any sense!", rather than something like, "no one would fall in love at first sight with a fatty like you".
Yeah that's probably the case but those sort of words are what stay with people (but now I'm talking more real life than manga), when a friend says something cruel unintentionally, it hurts a bit more because you think about how that's what they must be thinking on the inside. Either that or it was translated wrong and he didn't say it in such a horrible way haha but yeah, he might be a good friend but has a lousy way with words.

"It is a bit weird that he could fall in love at first sight with your appearance" what kind of a fucking friend says that...