Do guys get offended by this?

Your friend may be sensitive about his weight, but it obviously didn't matter to you before, don't make it an issue now. He probably enjoys being around you because it is not an issue. And he may have a crush on you. I think calling someone fat hurts regardless of gender, but you keep hang out with him and being his friend as long as he is one of the good guys. Guys don't usually like to bear their hurts or insecurities, particularly with girls. So, I would not mention it. If it happened again, it just remark that the frat boy is drunk.