I am actually asking on behalf of someone else.
He's been seeing someone for a couple of months, who is still really good friends with her ex and hangs out with him all the time. As far as I am aware there are no children involved; they just broke up and still see each other regularly. Friend feels disrespected and that it's odd that she's still hanging around with her ex so much, and that it adds extra baggage to the relationship. I think he's also worried that she still has feelings for this ex, which is obviously a bit crapola for him.
So, he thinks he should start hanging out with his ex (who he broke off contact with, thinking it's unfair on the new partner to have something like that hanging around) and/or break up with new GF.
What's the MN consensus? I think he should actually talk to her about it, and tell her that it makes him feel uncomfortable, and maybe ask how she would feel if the situation was reversed. He thinks that she'll complain that he's trying to be controlling (which I can also see).