I can't really put a take on this as there is a lot of background information missing. For example, do they have kids together? Was he seeing you before the divorce? How long ago did they divorce in comparison with him seeing you? Do you have kids with him?
And whether I know that information or not, I think that is his choice to be friends with her or not. It isn't your place to control who he wants in his life as long as your needs are being met by him.