Yeah, it sucks. I was in N's position at one point, and I just decided that I was going to be poly and if S (mine was an "S" too!) wanted to be with me, that was great, if not, that was unfortunate but I understood. She started dating around and now is with a great guy who wants to be mono with her. Sucks for me, but everyone has to follow their own happiness. I miss her.
Wish I had a "K" to help with the heartache.
It's basically N's problem. He's the one that has a decision to make. As for your friendship with S, it sounds like that's over. So as has been said already, comfort each other as best as you can, that's all you can do.
Sorry, didn't see your last post there, about her leaving and looking to salvage the friendship with S. I sincerely hope it works out, but I think you should both be reminded that she tried to be poly when it went against her nature, and this may be a similar situation. She may want to keep the friendship but not be able to, and she needs to know that being honest with herself and you about this is very important. I would suggest thinking about taking a break from each other for a while and then picking things up slowly and naturally.