Its a personal matter... I am sure, he does NOT even want to be seen either.... because he is embarrassed. At least, I hope so.
BUT it is still a private matter... and no one really knows, their circumstances... except them.
Everyone, sometimes, deserves a little non-judgment.
You don't know.... what they are going through.
He may not even know, that YOU know.... thus, HIS privacy was broken and invaded, too.
My Dad... once had an affair. My Dad... is my Hero and always will be. He was an OUTSTANDING man... and human being. He... made a mistake. He.... and my Mom, had problems, Which I was well aware of even as a very young child. I "understood" my Dad... and what he did. I NEVER EVER, held it against him. I LOVED him, always. I knew... he was only a flawed human Man... like many people. My Dad, passed away about 10 years ago. I NEVER EVER, held that against him... and to me... he is not "tarnished" nor a "jerk."
My Mom... if I may say so... was a very difficult.... woman. Very cold and distant. It is not anyone's "fault"... but a reflection of a weakened and lonely man.... at least for my Dad. I understood that, even as a young child. My Dad, TRIED, many things for my Mom... and was really a gentleman who did ANYTHING for my Mom. My Mom... did not really show, any emotion, toward him, hardly. My Dad, even sent themselves on a cruise, FOR my Mom... and still, she just put him down. Never really, complimenting him. She would say how my Dad never did anything for her. BUT, he built her a HUGE house, made lots of money for "them"... was very successful and attributed his success to her... he NEVER talked bad about my Mom and would always defend her. My Dad... was lonely. I do not, 'judge' him. He is and was... a STELLAR man... and the most genuine and loving and understanding Dad, a child could ever have.
My parents did not divorce over it.
Your friend's Husband, is not a "criminal." LET them, work it out... on their own... without an audience.... or even weird eye contact at him.
all the best,
Susan