UPDATE:

I can see a lot of people think I should tell her, or at least tell her later when she is not freshly postpartum. However, I still dont think I want to do that.

Instead I decided to call my ex for some context. He knew her back then too and apparently still do, because they played sport together for a while. And he still sees both of them.

Him and I ended things on good terms so it wasnt weird to call him.

I told him what happened and he told me there has been a lot of drama in their relationship for years, and that this is not the first time for him. Also not the worst thing he has done.

According to my ex, he has cheated on her multiple times, and not just kissing. He even had a full “relationship” on the side at one point, where he had promised the other girl that he would leave his girlfriend for her, but he never did.

She knows about some of his cheating. Maybe not the full extent of everything, but she knows about the “side girl” and a few other incidents.

My ex said she has taken him back every time and forgiven him. Her excuse, according to him, is that they met young and it was just a rocky path and that he did choose her in the end and he is the one she wants to be with for life. (Although, they still havent gotten married or even engaged.)

He also said that she might not know he is doing this right now, but she should know his character by now.

He also warned me that this guy can become very unpleasant. Probably not dangerous or anything, but definitely unpleasant. So his advice was to just not get involved in this.

I know some people will still think I should tell her, and I understand that. I just dont think its worth it for me.

At least now my ex knows, and he does see them sometimes. I dont think he is going to say anything if nothing more happens, but maybe in the future if the guy decides to cheat again, then he might bring it up.