Sarah is an evil repugnant bitch.
She has approx 3.5 billion, BILLION men to choose from on the planet but no she picks her very close friend's husband.
She's a marriage wrecker - in that she doesn't want a committed long term relationship with anyone, she just wants casual and you can pretty much guarantee that once she gets John she won't want him anyway, the thrill of an affair is always the illicit secret, the person you 'wanted to be' rather than the 'couple who are now together out in the open'. Affairs are never about the sex, it's about the people wanting to be something other than what they are now, a different version of themselves. Then when the outing comes, often the relationship fails for lack of sizzle and excitement. They are both severely deluding themselves. She doesn't really want him. She is emotionally crippled so much so that she only wants what other people already have and finds some sort of validation in having someone else's "goods".
John is a total dick as well. He is equally to blame. He is an evil repugnant man who should know better than to cheat on his wife. What is he thinking? To break the hearts of his children and his wife. To break their world apart - for what - sex?? A few moments of feeling fantastic for a lifetime of family happiness?
I would
a) tell Sarah I couldn't be her friend anymore while this was going on. Remind her that she has over 3 billion other men to choose from. Tell her to end it or you will be telling Jon.
Remind her she is about to nuke a very nice close friendship circle that has existed for nearly 2 decades due to her emotional and moral incompetence.
b) move to plan B if she won't and tell John to end it and if he doesn't you'll be telling Emma.
If they both won't tell Emma, then you move to plan C which is to tell Emma and watch the whole sorry mess explode in front of your eyes.
Your friend is no friend. She's emotionally and morally bankrupt.
She could have chosen anyone ANYONE else - but she chose someone else's man in her friendship group.
Pure evil, both of them, in my view.
Yes tell your husband and seek his advice. I would also be very very worried she'd shagged him too. It sounds like she validates herself by seeing how many of her friends' husbands she can bag.
Sarah would be no friend of mine. If she can do that to Emma, she can do that to you too, and may well have.