I'm curious if she was doing this before you guys got together?

If so, it's quite possible that K just likes to go out and have quite a bit of promiscuous sex, but she doesn't feel comfortable hitting on guys. Thus J does it for her. Yeah, there's probably quite a bit going on mentally if this is the case, but casual sex isn't too uncommon for young people. It's also possible that J started disliking you due to conversations with K, where K confided in her friend the feelings that eventually led her to break up with you. (See what a funny thing perspective can be?) If this is the case, then my guess would be that she really ended the relationship because she wasn't ready for something that in-depth and monogamous. Lord knows what she believes though.

Anyway, I don't know anyone involved so I can't say with any bit of surety what's going on. I'm just providing another possible explanation. It's very difficult to gauge people's intentions without open and honest conversation, which is the route I'd probably take with her. Promiscuous sex with strangers can be very self-destructive, so even as a friend you'd have right to be concerned. But jumping in to play white knight, hurling accusations at her friend, and judging the situation without full understanding of it could easily drive her away completely.