His betrayal is heinous, yours is supreme. Do the right thing.
I would feel really uncomfortable under these circumstances. I think I would be inclined to express my sympathies about her marital difficulties and let her take it from there. If she wanted to know more and asked me what I knew I would tell her the truth. If she is happy to know no more than she does now I would drop it. But I don’t think I could just sit there talking to her pretending that I don’t know anything.