Have you ever grassed on an affair?

[RedKite1985]

As above really.

I can't bare to witness the smugness and disgusting way someone is being treated

It's never ended well either, I've been blamed twice for telling but it wasn't me, I was just vocal about my disapproving so I seemed the likely candidate.

I've always been amazed at how the adulterers seem more concerned with the snitcher. Rather than thinking 'if I'd not done it there would be nothing to tell'.

Whatever you chose will be wrong op, all I can advise is think how it will blow up for you. If she's as loose lipped (again seems another common trait) it will come out in the end.

Dragonwithagirltattoo, reductio ad absurdum.

I did tell a friend. It caused a load of aggro. I wouldn't wade in to the middle of somebody else's drama without a good reason. Friendship and loyalty are good reasons obviously.

No. In both cases, I felt more sympathy for the cheating partner than for the one being cheated on.

What would be the kindest way to tell someone?

Yes.
Don't regret it.

Yeah and ended up splitting my parents up but hay ho.

A load of people kept my exes affairs secret. The old 'nothing to do with me' despite them being my friends.

No because the person involved has children.
It's none of my business and for all I know they could have an open relationship.
But every situation is different.
Would I want to know? Honest answer would have to be. I'm not really sure.

Whether you divulge or not only you can make that judgement but you don't need anyone to tell you. That he/she might not thankyou, and might blame you.