I'm sure I'm stating the obvious to most on here but this kind of behavior is usually a desperate, flailing, attempt to get involved in the interaction. It comes from a place of neediness.

A mate see's you in an interaction with some hbs. He doesn't have the balls to open himself. He is acutely aware he is standing like a chode while you are doing what he wishes he could.

He then works up half a testicle ,comes over, and tries to be high status and funny, by bagging you out to the girls.

Of course you now both look bad.

It would be a lot better all round if he just came over and said "Hi". However that would be an honest, putting your ego on the line, commitment to the interaction. It's much easier and risks less to wander over acting like you don't care, like you are above the situation, and take the piss.

To be honest I got this a lot in the past but very rarely anymore, my close friends wouldn't do it, they're not perfect but are past the point of pulling shit like this. I think if it happened I would say to the guy straight out "Look mate, if you want to come and chat to the girls with me that's great but don't fuck up my shit". They'll act like they don't know what you're talking about but I would hope it would get the point across. Remember, if you're opening they want to be in your good books.

If this didn't work they would have to get a bit of an ego damaging talking to.