But if you don't know the wife, then you cannot possibly know what is going on within the family. Many years ago I knew someone who had an affair within my friendship group. We all made it abundantly clear what we thought about it, both to her and him.
Not long after that the friendship group drifted apart purely through circumstance and we lost touch. it wasn't until years later that it emerged that the woman was in fact in a violent marriage, and her affair had been her escape (she did not stay with om). Had anyone in that group seen fit to tell her husband (who none of us knew) she could well have been in danger. Now some might argue that she brought that on herself by having the affair in the first place, but the reality is that life just isn't that black and white.
There have been posters on mn who have been in dv situations and who have had affairs which helped them leave. And while an affair is never ideal, it is also not as straightforward as some see it.
For me personally, I wouldn't get involved in so much as that I wouldn't cover for them, wouldn't go out with them as a "couple," or help them in any way such as lending them my house (for example) and if the wife (or husband) who was being cheated on was a friend I would probably tell them.
But if the partner wasn't someone I knew in any way I wouldn't tell them on the basis I know nothing about them or the relationship they are in.
To compare knowing about an affair to knowing about child abuse or domestic violence is ridiculous.