this is a tricky one really, looking at it one way you cant tell him who to be friends with and he has the right to remain friends with whoever, tho looking at it another way... he should respect your feelings over an ex and if it bothers you stop seeing her.
i have a certain view on this........ me and my partner split up for nearly 2 years, he met someone else and even moved in with her, but he was still coming round to mine asking me back, this is slighty different as we have a child together so he told her he was coming to see his daughter, at the time i didnt want him back but it didnt stop him trying it on!!! we did get back together in the end and we are happy with 2 kids now, this was years ago, so it my mind you have to be careful of the ex's, there were feelings there once, there could be again, or another way to look at it is, if they wanted to be together, well they would, but their not,
x