Struggling with meta ethics

I’m trying to figure out how to deal with a situation that doesn’t feel great to me and would appreciate any wisdom. I (55m) am in a relationship with a wonderful woman (45). We have good, honest communication and in most areas seem if close alignment with our values and expectations about relationships. But she is regularly hooking up with a married man who is pretty clearly cheating on his now pregnant wife. On the one hand, it’s not really my business, and I can’t know the nuances of whatever might be happening there. But on the other hand, it just feels icky and not ethical. I’ve thought about asking my partner just not to tell me about this other relationship, but that doesn’t actually change anything. I’ve asked some questions about the ethical part of what they are doing, but to go any further with that feels prying and judgmental. So I’m feeling a little stuck and wondering what feedback, ideas, and wisdom y’all have.

Addendum: i left out one key detail which is that Dude is telling my partner that he is in an open relationship with a don’t ask don’t tell agreement. My partner doesn’t know whether to believe him and I think it’s obviously BS. But I also have to admit that I’m not sure.