I can't believe I am going to say this but here goes.

I would just try avoiding it. Try to get him to set up a specific time and so on that he has to deal with her and just don't be around when it is happening. If your not directly dealing with it you might not be bothered so much by it. Other than that just have him keep the topic of her to a minimum as much as possible. Out of site, out of mind, or so I hope.

Well, that is one path anyway. I can't say that it is the best but it might work. I really hope someone comes up with a better solution than this but it might be worth a try.