My situation is very similar to yours with your ex. I have asked her why she is still friends with her ex considering all the crap she talks about him. She just tells me that she doesn't know why which is fucking strange. Sometimes I have asked her to invite her ex over so that we can all play board games together if she really wants to hang out with him. Her response has always been something like she wouldn't like to hang out together as she wants to stop talking to him completely.
It's all a bit strange to me tbh. I have never been in this situation before and I am certainly not the kind of guy who likes to control my partners.