Do I tell her about the affair?

I would ‘stay out of it but consider if something ‘anonymous’ could be done. It wouldn’t sit well with me either and I would definitely have distanced myself too

My father treated my mother like this for all of their marriage. She has always said the worst thing was finding out that everybody else knew.

Yes 100% tell her, however be prepared for them to cut you off socially and business wise

I mean I’d rather know even if it was from an anonymous person. The anonymity suggestion was due to the impact it may have on her husband/business in this case. I’d much rather be told that way than have everyone knowing and not telling me.

Not really. I think everyone being fine with knowing and playing along is creepy and unsettling. There really are women who would never pick up on anything because they truly believe it's impossible that their partner would be so callous because they would never cheat themselves.

The advantage of anonymity is that it enables the sender to break the news as thoughtfully as possible - OP can pretend to have talked to whoever caught them in the act rather than heard it via gossip because everybody knows. She can always come forward later.

The fact that women have come on here saying they would have appreciated someone telling them makes me think she has to be told. It's up to her what to do with it.

This doesn't make sense. If you're happy not to know about it the you must not care if your husband has an affair. So then what difference does it make if you're told or not?

Do not get involved.

I agree with Bilbobagginsbollox.

Unless you've changed a lot of the details I think you've given enough information in your first post for those involved and the wider family to identify themselves.

How do you know some of them are not on MN?

Anonymous contact is so creepy and unsettling I wish people would stop suggesting it..so it's fine to take this secret off your chest ans carry on as you are meanwhile like a coward you drop a bombshell on someone and they don't even know who knows, who is watching, who is pitying. You make them paranoid on top of it all, just selfish and thoughtless advice.

Please tell her. Having been in a similar situation myself, I am forever grateful to the person who told me. A lot of people knew and didn’t say a word, which just added to my feeling of betrayal.

“It’s none of my business.”

Correct. There’s your answer. Keep out of it.

This is always suggested on this kind of thread because people know telling is a thing that can backfire in many ways. If you need to stoop to doing something anonymously, it shows you know you shouldn't be doing it.

Posts like these just highlight what a selfish world we live in.
Look after number one and to hell with everyone else.

Not everyone is so inclined to play moral crusader and take it upon themselves to ‘hold
people accountable’ for immorality (not criminality).

For those that are so inclined, it’s ridiculous to suggest that commitment to this role should override any sense of self preservation, as if the warm fuzzies from ‘doing the right thing’ are adequate compensation for blowing up your own family and business (and OP very much risks both), whilst the moral criminal you’re attempting to punish likely walks off whistling and unscathed.

With so much detailed information in this post, she probably knows by now!

it’s one thing sharing information about yourself, but so much on other people can’t be a good idea (unless you have changed a lot of details).

No, i absolutely wouldn't want someone to tell me, thereby demonstrating that they think they have a right to interfere in my marriage.

Any sensible person would put an anonymous letter straight in the bin, treating it with the contempt it deserves.

This is nothing to do with you, OP. Never interfere in other people's relationships, because you have no idea of the true situation. Interference just makes you look like a gossip and an unpleasant person.

Mind your business and let them do what they need to do. Nothing to do with your moral standing and if you say something it will backfire spectacularly

The OP is not a close friend and she has no idea what is actually going on in this relationship. The idea that she should send an anonymous message is horrible and totally irresponsible as who knows what effect it could have? I can understand it would be a dilemma if she was a confidante but as it stands I really find the idea that she has a “moral duty” inappropriate.

Unless you can say to her there was gossip going around we were involved in and now I am putting this gossip onto you then no i would not say anything

I agree to an extent, of course, it will always happen, in the same sense that there will always be people who commit crimes, etc., etc. However, by not challenging it, you are enabling it and giving the impression that it's ok, it's not. It will happen, yes, but should they be let off the hook, no.
Knowing about it and taking your view also makes your morals and standards questionable, or does it make you an easy target for people who behave like this, like, 'oh dont worry, they'll not say or do anything about it, just carry on and take the piss out of them, they just let you because they just think it happens and its part of life, they let you get away with anythung'.
Yes, you do need to accept that this stuff happens, but you shouldn't roll over for it, you still need a backbone to let them know they are taking the piss and you won't bow down to it.

The OP doesn't have proof, though, her husband's friend says so. I wouldn't report something I'd heard, if I'd seen something it might make me feel a bit differently but even then, unless it was unequivocal, perhaps not. Passing on rumours is an odd thing to do, and I'm not sure it will be as convincing to this lady as you think.

Op, I would tell her, but I would do it anonymously. I don’t know what any of your businesses are but I’d worry tha the husband badmouths you to other clients even if you think you can afford to lose your business from him.

I think the psychological damage of an affair is the worst thing. I’ve seen enough threads on mumsnet to know that she may have doubts and he may be gaslighting her or denying it. She might be thinking she’s mad. He might be being mean to her at home, they might be trying for a baby…

if it were me I’d want to know.

As other posters said, you don’t have evidence and he will deny. She / They would think negative of you. You get nothing out of telling her in the end.

It's her husband who heard rumours from someone else like is it third hand account? If anyone who has more of a moral duty after the ones who actually caught them is surely the husband who heard the gossip, you're even further away from the rumour to take it on as your duty!!! It sounds like there is jealousy of the OW or the wife and a desire to watch them suffer.