It's a good thing I saw this. You've come to the right place. I used to have an "Ask MULLY" forum where people could ask me questions, these types, and I'd give the answers they were seeking. Kind of like Ann Landers with an attitude.
Anyway, here's the deal. First, what kind of relationship do you have with your ex? Was the the split amicable? Did you part on halfway decent terms? If so your first move is to give her a call and say "Look, I'm not going to tell you that you can't play in this league but I would appreciate it if you didn't. I'm just not in the place right now where we can hang out comfortably."
If it wasn't very amicable.....you're screwed. She'll do it just to spite you.
The one thing that sticks out to me in this entire thread is that everyone is pointing the finger at her. To me, I'd be pissed that the guy that you know invited her in the first place. I'm sorry, in the man code you just don't mess with a friend/acquaintances ex wife/girlfriend. It's just not done. I think it's a shi++y move on that dude's part and I'd be pretty close to wanting to whoop that ass. She didn't do anything wrong, other than accepting his invitation....which could be a stab at you, but still, that dude is wrong in all sorts of ways for even contacting her. I would definitely confront him about this. I don't know if you're the type to take that approach, which is all fine and dandy, but if it was me I'd be talking to his ass real quick like.
MULLY