Maybe discuss it with your friend? I wouldn't expect any sympathy from your ex, I'd guess as far as he sees it, you have nothing to do it (well, that's the facade he'll put up at least, even if he doesn't feel great about what he's doing). She should understand where you are coming from and even if she won't break up with him (after all, there's no reason why she can't date him) she may be more careful around you and take care not to rub it in and things.
I can understand you being hurt, but take a step back, and look at it that you and him didn't work out, you've moved on and he has the same right, just as your friend has the right to be happy as well. It'll take some getting used to, but it's something you'll grow to accept. In the meantime, if you friend is understanding about it, that'd help a whole lot.