I am wondering, do you have any hard evidence that S wants to break up you and K? It sounds like there has been a lot of virtual interaction. Facebook keeps your messages for quite a while. Is there anything that you can show K that proves that S desires to do this?
If K has written proof that S wants to break the two of you up, and still wants to stay with her, then you can make an informed decision about where you want the relationship to go with HIM, if anywhere.
I personally believe that if my partner wants to persist in a relationship with someone who doesn't respect the relationship I have with him, who actively seeks to undermine and end our relationship, then my partner is disrespecting me by keeping this person in his life. I know that is not a popular point of view in poly circles, but that is where I have arrived at, after a few metamours that were so toxic I can't even write about them without getting a fit of PTSD.
I can see that you wouldn't want veto power now, but if you end up having kids with K, you may want to reconsider that.
Female, married just about forever to Scout. My boyfriends are E. and C. Both are married, and C also has another lover, J.