To not tell my friend that I’ve heard a rumour that her DH is having an affair?

Everyone always says to tell them because they'd want to know but the truth is sometimes they don't want to know and the messenger gets shot. Many years ago I found out my friends DH was having an affair and I told her. She never spoke to be ever again because her DH managed to convince her I was just jealous of them and wanted to split them up. So no, having been there I wouldn't tell them because sometimes you get shat on from a great height for doing it and lose friends.

No stay out of it
but when it does come to light the you cannot let on that you knew.

If one of my friends knew something like this and didn’t me I would never forgive them.

I also wouldn’t sssume she won’t find out elsewhere, it sounds like OW is being very indiscreet and nail woman blabbing to anyone and everyone who knows how many people will hear

This is tricky the fact it’s over and sounds like he is refocusing on his marriage telling her might totally scupper things. I think the minor celebrity thing and people talking about it makes it worse though. I’m of the I’d rather not know…but if lots of people were talking I’d feel humiliated and that would change it for me. Could you start a hypothetical discussion about another friend and whether to tell her and see what your friend thinks?

Her friend doesn't know. She's heard a bit of tittle tattle.

Stay out of it.

I would be gutted if I found out my husband was having an affair, but I'd be even more gutted if I found out that my friend knew all along and chose to keep it from me!

Unfortunately, exactly this. Especially given the source and it actually just being unsubstantiated gossip at the end of the day.
There is no evidence and even if she asks him about it, you’ll be the friend that is a stirrer and can’t be trusted in the narrative he weaves.
Sadly, even with evidence some women will choose to believe or stay with a cheater, and if it’s their friend who revealed the infidelity they will sacrifice the friendship instead to maintain the relationship. All that will happen is you will lose a friend who is likely to need you in the future when her marriage breaks down.

I feel awful saying this as I would definitely want to know but no, I wouldn't say anything. As it's third hand, unverified and with 2 young kids, it's unlikely she'll leave now and you'll be seen as a gossip who tried to break her marriage up. I've seen it happen to a few friends.

I think I'd have to tell her, or at least give her the option to hear it. "Friend, when I was having my nails done this week, I heard a rumour about your husband. I don't have any way of knowing if it's true, but I felt I owed it to you to mention it. If you don't want to hear it, I understand and won't say another word."

Mind your own business no good will come of it.

Stay out of it unless you want to get shot, everyone shoots the messenger.

No don't tell her

It's gossip. You don't have 'evidence' to substantiate it. If you tell her, you as the messenger will definitely be shot.

Either do more 'digging' or stay silent.

If I were you OP:

I would have told my nail tech there and then “Oh, X is my friend, so I’m going to have to tell her about these rumours”. Obviously hindsight is a wonderful thing.

Then immediate after the appointment, ring friend and tell her exactly what the nail tech told you.

She needs to know. Real friends don’t keep these things from each other.

A ‘friend’ of mine who discusses rumours about my relationship behind my back isn’t a friend to me.

You don’t need to do any ‘digging’. You simply need to tell your friend what you have heard about her H. It shouldn’t be this complicated.

No, stay out of it. If you tell her you force her to do something about it - whether the rumours are true or not.

I would try to arrange meeting up with her soon and do it face to face. I'd say listen, I need to run something by you and I feel really wick about it but I think the right thing to do is to tell you. When I was at the nail salon xyz happened. I'm very aware it's not my business and I didn't share in any gossip but I felt it was best to come to you directly because that's what I'd want you to do if it were reversed. I've no idea if it's true or not that's all I know.

I would be direct, keep it very factual and tell her exactly what happened and what was said without speculation or your own opinion. She might already know, she might be upset. Just ground yourself and brace for whatever reaction she has. If she's upset at you then you know really she's upset at the info.

Really tricky one for you op.

How is this helpful? Really? Because if it's true then you are giving the husband the opportunity to deliver the message directly and cut out the gossip. If it were me I'd rather have that than be told by someone else. If it's not true then the wrong is as much against the man as the woman

How is this helpful if OP isn’t 100% sure he is having an affair ?

Could you mention it to her husband and see what happens?

It sounds like they’ve already split and for all you know your friends move was precipitated by the affair and she may already know and not want it to be public knowledge. Be supportive and say nothing. I’d be more tempted to speak to your friends DH than to say anything to her.

Dreadful gossip all round.

You should have shut the conversation down way way before details and names were bandied. Whoever it was about.

Gossip is gossip. You have no idea whether this is true.

And due to gossip there is frequently smoke without fire.

I’d keep out of it. It’s all vague and unverified, and just because your friend has been through a rough patch lately it doesn’t mean he’s having an affair. I’d wait and see if something more tangible turns up - if not keep it to yourself and remember ‘shoot the messenger’.