Originally Posted by Ltmusicdude
I may be completely off base here but it honestly sounds like hes envious of the relationship you have with your OSO (btw if i get any of lingo on any of my posts wrong please correct me). As a temporary measure you may want to try limiting the interaction you have with your OSO around him. I know that you absolutely should not have to do this but as a temporary fix it may help more than you think. You or someone else should also tell him that he won't find happiness if he is looking for it, if he really wants somebody else to like him he needs to just relax and focus on being him, not being him AT somebody else; it will make him come off as less desperate and he will have a lot more luck.
It may be true that he's envious, but I really don't think that's the problem. His behaviour is no different than a single guy who can't get a date and exhibits the same tendencies (hitting on every single girl he meets, complaining that he can't get a date, etc.)
This isn't about her or her OSO. It's 100% about him. As such, there's only so much she can do, and it sounds like she's done it. At some point, he just needs to realize that he's creating his own situation. By being miserable and desperate all the time, he portrays a front that no woman would want to date. Nothing sends women running faster than desperation (well, a few things, but it's right up there on the list). We can smell it from miles away. When someone is desperate, you never feel like they're interested in you as a person, but rather you as a token. Yech.
At this point, I think the only way to move him along is for everyone he knows to start making it explicit and obvious. Your friends aren't doing him any favours by clamming up about why you're invited and he's not. Of course it will hurt his feelings, but he'll recover.
A separation might help you out in that you won't be exposed to it anymore, but don't kid yourself that it will "help" him or make him see how he's acting. He's learned to externalize the cause of his feelings, which means leaving him will be perceived as just one more thing that is done "to" him. But you need to do what you need to do.