It gets even worse when you do that to a stranger in Wal-Mart. Yeah, she's a fucking whale, but I highly doubt telling her, "you're really fat, I can help you lose weight" is really going to get you a good response or make her want to lose weight.
Again, it's all about communication. If you really want to help that guy, you'll take the effort to effectively open him up to your feedback. Otherwise, it's a subtle insult coupled with charity advice from a stranger. Nobody wants that.