Have you ever witnessed an affair?

I’m curious about people’s opinions on infidelity.
It is not someone I know well, but I do know that said someone is having an affair.
Personally, I wouldn’t involve myself in letting the wife know.
That’s not my business, nor my responsibility. I’m sure she knows deep down. It seems to have happened at least once before.
I felt nauseous seeing it. There’s something grossly seedy about it in my opinion