What an utterly shit situation to be in, OP - full sympathy.
Here you are genuinely in turmoil. Whereas the cheating man swans round without a care in the world.
In terms of saying anything, you are damned if you do, and damned if you don’t.
Everyone always says they’d want to know (fair play to the person upthread who said they wouldn’t). That’s not the truth, though. Far from it.
Most people, on finding out, shoot the messenger. They just do. AND, they simultaneously turn their humiliation and anger on all the people who knew, but kept their mouths shut.
As I say, it’s a completely shit situation to be put in - all the while, the cheater is fully protected.
No advice, I’m afraid. There’s zero way of knowing whether your poor friend would want to know, or if she’d cut you off forever to protect her husband.
The only approach I’d venture suggesting, would be to tell the cheater you know (or get someone else to - I doubt any of the men would be willing, though), and urge him to come clean.
Even if he doesn’t, if he realises some of his wife’s friends know, it might force him to tone things down a bit.