She didn't. They have two kids together (last I heard).
What happened that night was that after giving my quick and final goodbye to my former friend, I walked out and then just had to be by myself and contemplate how horrible the situation was, for the night.
I had been correct to end that former friendship immediately (because what I said was true -- I could no longer bear to even look at that bitch), but how to tell someone about what I had witnessed was far more complicated.
So I did tell Sarah (fake name) the next day about what happened, and why I had been gone when she got back from buying snacks.
She believed me. We all knew my former friend who fucked Sarah's boyfriend (let's call immoral former friend Elle) was not a good person. This bathroom-fucking was, unfortunately, not a surprise to Sarah, who had terrible taste in men anyway.
But "She loved him."
And there you go.
The friend group dissolved immediately after I decided never to see or speak to Elle again. She had actually been the (fun-times, woohoo!) center of it, but she had always always been our groups missing stair. (Google "relationships missing stair" for more context.) You know, her shit? We ignored it.
But we had all become friends as kids, for stupid reasons. And most of us (well, half of us) were finally growing up when Elle did this.
I didn't even tell anyone aside from Sarah. I don't think Sarah did either, unfortunately. I kept it to myself because I viewed it as her own personal business, when it came to how to handle it. She did dump Elle as a friend. But she had two kids with the guy.
But apparently I had a reputation of being both decent and tolerant, and so when I was like, "Oh, Elle will be there? Then I won't," about all future gatherings, and when people asked me about Elle and I said I had no idea, they realized she must have done something awful.
So this friend group full of people who were bonded by a shared history, and by all being a little crazy when we started off, shattered. As it should have. As it needed to.
I made other friends.
So there's the story.
(Jesus. I don't think I've ever explained this so clearly to anyone else before.)