Funguy I personally would not tell WHY because I dont want to be the one to destroy a family, even though the husband is going a wrong thing.
I had an incident in my family, my cousin tried to do something some years ago, I have not told his wife, who is my cousin also, as they seem happy together and have kids. I have forgatten but not forgiven.
Another incident a husband of my friend tried to invite me for a coffee, I declined but didnt tell anyone, as why destroy someone's life, things dont stay hidden for ever, if he is cheating on her, she will find it out someday and he will get his punishment if not here then in the next world.
It's not you destroying the family. It's the husband.