I would totally butt in personally, cheating and having affairs destroys the integrity of everyone who knows about them in my opinion. I would be very concerned if this were my husband.
I have been involved with a man who turned out to be having an affair, he told me he wasn't, I have also cheated myself and my boyfriend Mono destroyed 17 years of marriage because of an affair. I can't stress enough how destructive they are. I was with the cheating man a year ago and it is STILL coming back to haunt me and I didn't even know!!! I will never see anyone without checking with his their partners first as a result.
Sure there are times when people get through them and are able to construct some semblance of a relationship again, but you ask anyone that has ever been the one cheated on, something dies inside and never returns. That trust never comes back entirely. It carries on from relationship to relationship and the person seems to never entirely be present in their relationships again as they find it hard to fully trust again. Why would he want to be a part of that process of destroying that in someone else? Has he thought of how he would feel?
Your husband, regardless of the casualness of his intentions, is treading on a very fine line. He has become involved in something that is destructive to another person and another mans family. Her husband will be damaged and so will her family because of their behaviour and so will she. Why would he want to be a part of that?
I would be strongly advocating for him to end it. If I were him I would tell her that although she is a good match for him she needs to negotiate with her husband about seeing other people. She can't do that with a boyfriend in tow if it is going to be a good conversation. Boyfriends complicate things. I would leave her to it and tell her that when she has something sorted out that he will be available to meet her husband and start again.
Of course he just wants this to be casual, so it might not be worth it. In that case you seem to have an "open" relationship rather than poly, and there are some differences with that. The level of caring,love and commitment to their entire lives is not there. In this case I would encourage him to seek out women that are not cheating and leave her to her cheating ways....
There are a lot of good links to threads on here where we have discussed this topic at length. If you were to do a search on here for cheating and affairs you will find them. They could give some valuable insight.
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