Thank you for your replies. You have some very valid points.
Let me start by saying , he is not a bad person. He just has a ton of emotional issues. Sometimes I do feel like I am enabling him in bad behavior.
However, I realistically realize I can't make his decisions for him. I can't make him tell her he is not monogomous.
We are getting together this weekend. I think I will discuss with him how I feel and encourage him to come clean with her.
This is my first truly poly relationship that was more than just an online emotional relationship so I'm treading on uncharted waters . I am learning every day what boundries I need.
I truly don't want to 'break up' with him. We have had a deep emotional connection for over 15 years (even when he was married) so it's not as easy as it sounds. However, the sexual relationship has come and gone at various times over the years. It is just something I feel I need to do for my own piece of mind.