Didn't tell friend that I have slept with new man

Agree with others, it won't sound great if you bring it up now, at a point where the two of them are doing so well together. You sound absolutely lovely, but it might give the impression you're a bit jealous and want to cause trouble (I know that's not the case)
To me, it's actually better that you hadn't said anything if she did ever find out, as it suggests you didn't think it was that important! If she does find out I wouldn't lie, I'd just explain it as you have done here. I think if you make out you thought you'd told her she will know that's not true

It sounds like they are serious about one another.
He will likely have mentioned it to her over some drinks, they’d had a chuckle about it, and then moved on. Because it didn’t mean anything to him, your friend isn’t bothered and so hasn’t mentioned it to you.

So I’d just leave it

Exactly fkn shit advice or what.

Completely being nosy here just out of pure interest, How did you end up suddenly not sleeping with him? Did something actually happen? Did you have a conversation? Or did you just suddenly stop having sex?

Not if both friends see it as nothing and it was a short fling.. why tell her something that just doesn't matter anymore silly advice.

No I dont tnink she needs to know ... why would she now.
If you had spoken as friends about it in the past then fine that way but I dont see the point in either of you bringing it up unless you want to chance spoiling your friendships they might not feel as easy going you will cause unease .

Perhaps he has already told her and because she knows it was not serious and meant nothing long term to him… she’s not bothered enough to mention it to you

Because they all know each other and she was the one to set them up so it's hardly outside of the realm of belief that he might at some point mention that they used to date? Not sure it requires it to be posed as dramatically as possible for effect, rather than just casually mentioned so that she knows and doesn't feel foolish when she eventually finds out.

It's not like it actually meant anything so why would it need to be proactively kept a secret by all parties. As I mentioned, people will view it differently and presumably would deal with it differently in real life as a result. If I started dating someone in that context I'd have mentioned it. Anyone with an ounce of maturity would just understand it for what it was and move on.

Why are you not thinking he’s told her
and because it’s of little interest to her - she’s not mentioned

I’d be the same in her shoes

Tough one @Freinddilemma both you and Tom might have thought it was the other person’s ‘secret’ to tell and then both left it and now it is almost too late. He won’t tell her because it would put you in an awkward position and you don’t because - it’s been so long it would be more unsettling for your friend. Yes, in hindsight, you should have told her, but, you can’t turn back the clock. I don’t know what to suggest - it might help to write down ‘what you would say if you were to explain why you didn’t say/why you left it’ but I still think keeping quiet might be the right thing now it has been so long … “arrgh” completely get how you feel though!

Is anyone else likely to tell her?

If not, I wouldn’t.

He might have told her
and it’s no big deal for her so not mentioned

This is so needlessly catty.

I'd tell her if she was a close friend I really cared about. A casual friend I rarely saw, I wouldn't bother. These kinds of things always come out eventually. If they're together years he'll probably tell her and she won't view you kindly.

regardless of should have, would have, you didn't. just block it and move on.

are you a little bit jealous you got a 4 month fling and she might be getting true love?

Don't say anything until you get to the Matron of Honour speech then drop it...😀

You didn't do anything wrong at all. It's not as if you are still harbouring feelings for Tom. I would just say nothing at all. It's not relevant to anything.

Yeah I wouldn't say "I thought I'd told you" because that's a lie and she'll probably realise that. And being dishonest will make her think you're hiding things.

Say nothing for now and if she finds out just tell the truth. You didn't tell her because there wasn't a natural time to do so and it wasn't a big deal because it was so brief and so long ago. That's all you need.

You don't have to tell your friends who you've slept with.

Old saying - It is aint broke dont fix it...

Why rock her boat?

don't ever pretend you told her and she's forgotten - someone wouldn't forget something like that and implying it would mean you are more than happy to put your need to settle your own feelings of discomfort over her feelings.

just leave it now and if it comes up be prepared with what you are going to say. In reality its only likely to come up if he tells her in anger one day.

I agree with PP about mentioning it now specifically might look like causing unnecessary drama. See how their relationship pans out. It may be the sort of thing that comes up over dinner and drinks in ten years' time, when you're all settled and secure in your own relationships.