I'm going to keep quiet about my friend cheating on his wife
Why would you keep quiet about him cheating?
He's my friend I want to keep our frindship.
It is good to stay out of other peoples romantic relationships. You should also not help them facilitate the cheating behavior that would be unethical.
Eh, I can't stop the cheating. I can only bite my lip and enjoy that he is cheating on his wife, and idgaf that he's breaking her heart.
I still think it's wrong that he's even cheating on her. I think the right thing to do would be to just come clean.