The matter of whether to tell OW’s H (and family) or not, is a thorny issue indeed.
Yes, morally it would seem that informing her H of her cheating ways would be the thing to do. Where and when does that end? How minor an issue before you ignore or overlook? And for who?
I think it depends upon if you know them. If OW and her H (and family) are completely unknown to you then do not involve yourself.
This is similar to why you don’t involve yourself or manipulate H’s path. He is in a journey and if you alter it, for good or bad, you take on responsibility for whatever future outcomes may transpire. And something always transpires. Our lives are all built upon the steps of our past.
OW and her family are also upon a journey. Yes, they may be headed for trouble. Or ruin. Or salvation. Do you want a hand in it?
If the person is known, a friend or family, you already have a hand in their life. Bring the affair to their attention.
Most times the OP is unknown and it is best to leave it be. Let their illicit relationship/affair run its course and burn out on its own. You don’t want them to feel “it is us against the world”, for it solidifies their relationship more. Fate usually is good at doing what is needed as it is needed.
I believe you already confronted H about his affair and told him you might spill the beans. He of course freaked, and panicked. I am sure OW and him have been on high alert listening and looking for signs.
I think, I’d let go of this idea of informing OW’s H. Leave this to fate. It will be uncovered when and how it will.
D
Feelings are fleeting. Be better, not bitter. Love the person, forgive the sin.