and she's chosen to reconcile with her husband, I think you have to accept that he will appear at group activities. You don't have to be super-warm and friendly to him -- you should exhibit basic courtesy, though. (A brief greeting) You can always talk to other people there instead of him.

You can also try to organize mom's-night-out type events too, so you can spend time with her without him. But as long as other women's partners are included, as long as she wants him there, he must be included.

To do anything else would be punishing her for

1. confiding in her friends

2. trying to keep her family together

3. trying to maintain her friendships

If you see him doing anything inappropriate, that's another matter, of course... but at the moment, all you know is that he committed a great offense against his wife, and she has chosen to try to work things out with him.