I can relate to your preferred flavor of poly, and that is why something rings of misunderstanding
if not and maybe it is because so much of what happened in your story is a black box of mystery to us who don't know you personally. But pride often gets in the way of understanding, especially when all parties involved view themselves as being wronged, or otherwise more in the right, and when until each side fully understands the truth of what happened, there isn't likely to be any resolution. I say "truth" because when things are black and white and one side of the dispute is the innocent victim, then one side tends to be more of a vindictive asshole, and if that's the case your husband would have to be the epitome of the stereotypical male people refer to when they call males "dogs"
and from your writing that isn't the impression I received in reading your words. I don't know what happened, or what bad information or wrong assumptions led to their relationship bringing such a great deal of pain and unpleasantness, but typically when it looks like the metamour is that cold and callous as well as unapologetic with no hints that anything is going to change the fact that your husband is still considering any sort of friendship or interaction is the tell tale sign of a misunderstanding.
a common one is not identifying the relationship as open or otherwise non-monogamous, which I have seen happen when poly-people's friends are not familiar with poly, a traditional friendship can that eventually becomes sexual leads to such misunderstandings. The worst type of misunderstanding happen when a non-monogamist denies their Primary relationship is "open", but that is the extreme end of the spectrum and only your husband, yourself, and the ex will be able to figure out where the misunderstanding took place