Mother inlaw and her lover

Hi all, so I created a new account because I don't want any of my family members to see that I posted this.

Well I'm married with two kids. I've had issues with mil boundary stomping. We've been able to make some improvement with that but it's been a long road. It took me a long time to be able to leave my children with her. Husband and I decided to leave our children with her for the first time for an over night stay because husband and I were going out of town to celebrate anniversary. Come to find out that while we were out she decided to bring her 'lover' around my kids. He's a married man. He's made it clear he isn't leaving his wife. I've never met him and don't plan on it. This is a relationship that is obviously not going anywhere. Dh made it clear to her that we do not want her bringing her lover around our kids. We don't know who this guy is and it makes it very uncomfortable for me. I don't agree with what she's doing at all. We don't know anything about this guy. I'm so upset. I found out that he was around my children because my child just told me that he went over while they were there. This happened two weeks ago by the way. Am I over reacting? Should I just let it be? I understand it's her life I just don't want it around my kids.