Weight issues?? WTF?? I don't have weight issues first of all. Not all women are self conscious about their weight/looks.
If I was talking about it ("it" being any subject I may be blabbing about, not just 'weight issues') in front of him, he's welcome to join the convo. Public conversations shouldn't be 'private', if I'm speaking about a certain topic in front of people, it's because I don't mind them hearing my thoughts. If I'm keeping something private, you're not gonna hear anything about it from me. I'm not fond of people who butt into my business, but if I'm talking about something in front of people, I know they can't help but over-hear. Sometimes I jump into random conversations I hear at my coffee shop. I'm never trying to be rude, and I've never had anyone tell me off for doing so. If I ever do offend anyone by adding to their conversation, I'd simply apologize and be on my way.