@LatteLoverLovesLattes
Talk to your husband and take it from there. I think it’s so conniving that she has told you not to tell him. You have more loyalty to your spouse than anyone else you have mentioned
@Jamboree01
This is none of her husbands business Keeping a friends confidence is fine, you don't have to tell your husband your friends business. No different than if Sarah had had a termination & wanted to keep it private. It's not being disloyal to your husband not to tell them other people's personal stuff.
This is a very different situation.
What Sarah is doing is wrong. She knows that as she has kept it secret from you, her best friend, for 4 years. Now she has decided she "had" to tell you she is asking you to keep it secret from your DH.
So now you, a totally innocent party, are left being the one not sleeping at night and worrying and stressing over the best thing to do, while she sleeps well dreaming of the wonderful new life she is about to have with this married man who is going to abandon his dw and dc to be with her.
You are damned if you do and damned if you don't. You can't know the best choice because there isn't one and because it involves other people whatever you choose has an impact on other people's lives.
Ok, so you know John. Is it at all possible that he was just having a "fling" but months and months of lockdown stuck inside with his DW and DC mean he's deluded about the easier life once he is with Sarah, who is child free and fun? While his DW is probably stressed and harassed homeschooling and running a house while he is probably distracted because all he can think about is Sarah? Is there a chance that once lockdown is over, he's back at work and the kids are back at school, that he will feel less like he needs to leave his DW?
If so you might be best just leaving it. As long as Sarah doesn't decide to tell Emma if John changes his mind!
Any option stinks for you. If you have a strong marriage and usually share everything (or most things!) I would be tempted to tell your DH. If you don't tell him the biggest impact for you could be him finding out in the future and being hurt or upset that you didn't tell him. You know your dh and what he is likely to think or feel if you don't tell him.
After that...how important is the bigger group to you compared to your friendship with Sarah? If Sarah is more important than the rest of them keep quiet. If not, is there anyone else in the group you can talk to?
Also if John leaves to be with Sarah, be prepared for Sarah to drop you as she is immersed in her lovely new relationship and being a step mum, and for her to come running back when things don't go so well.
I've been the friend who stood back when my best friend had an affair. I knew her dh but not well and I didn't know the other couple at all. It was a horrible situation to be in and I feel for you. Sarah has done what so many having an affair do...think only of themselves in telling you. I am no longer friends with my old best friend as she used me horribly during the whole thing for her own selfish desires and while she is still with her DH I just allowed the friendship to drift apart. I miss her but she became someone I didn't know or like very much.
So sorry this is so long but I really feel for you. This is an awful situation that she has put you in. Good luck with whatever you decide, even if that is just to say nothing...I'm which case maybe tell Sarah you know but you don't want to hear anything more about it so you can distance yourself a little.