How often does he speak to his sister? He needs to place boundaries there, too. Tell sister he isn't interested in hearing about anyone's infidelity.
"Oh, why do my actions have consequences?" ~ Homer Simpson
"She knew my one weakness: That I'm weak!" ~ Homer Simpson
Even the one person that left her husband for her affair partner, married him, and had babies with him; wished she did things very differently and told him so.
I Thank God for these people, actually, because they helped "lift the fog" for WS. I don't think he would have realized the immense wrong of the situation if he didn't hear these stories and what a train wreck each of the affairs left behind. It also made him realize he didn't want to be a "dirt bag", who screws around; so he's changed his outlook, demeaner, and has made huge strides in MC because he knows he needs it.
An unhealthy FWS, or a FWS with incredibly poor boundaries and still weak coping skills would continue to listen to the garbage that spews from those so deep in the fog that they can't see the train coming. It is likely a slippery slope at worst, or a path to a slippery slope at best for a FWS to be listening to the goings on of someone currently involved in an A.
I'm so glad my H is a healthier version of his former self as he has even cut off his own family members who are even involved in A's, whether they talked to him about them or not, he wanted nothing to do with them until they got their act together.
Every word doesitgetbettersaid. If your husband doesn't want this kind of culture around him he only has to lay down boundaries with those that do it to stop it dead.