This is the definition of a Clusterfxxk.
Sarah truly is lowlife.
But I think you have to be very careful.
It is going to blow up and to be quite honest I really wouldn't want to be around when it does.
When it comes out, Sarah, who clearly has zero loyalty to ANYONE, will throw you under a bus as someone who KNEW, and by keeping quiet, approved.
Expect to be very harshly judged by your shared circle
John is utter scum.
I feel very sorry for Emma and her children.
OP,
You strike me as both nice but unbelievably naive.
A decent man would NOT support what Sarah is doing, even if he liked her.
A decent man would be appalled, just as any decent woman would be.
If my husband was anything other than appalled I would be shocked and disappointed.
The absolutely worst part of this is that Sarah and Emma are close friends, without a doubt.
Disgusting, unforgivable behaviour.
I feel sorry for you, but you are going to be judged very harshly for keeping their secret.
There is something truly awful about a close friend going off with your husband.
It really breaks the heart of the spouse left behind.