Three of the friends had been friends for a long time. I'm not gonna use real names. Joe and Katie have been dating for 3 years. At the start of their dating, Joe was working in a different state. Their friend is Jack. While Joe was away at work, Katie and Jack slept together a few times without Joe knowing. Joe also cheated while he was away. 

Come back to current time and Jack now has a serious girlfriend named Jill. Jill quickly becomes friends with everyone in the friend group and her and Katie become close friends and share many secrets together. Everything is happy as can be. However, something soon happened that made the couples angry with each other. We hear from Katie and Joe that Jill is a w***e and a drama queen who told Joe that Katie slept with Jack while Joe and her were dating. After 3 years, Katie hadn't told Joe herself that she slept with Jack. Joe basically wrote Jack off as a friend and the two couples were no longer friends. 

My boyfriend and I took Katie and Joe's side when it all happened, but then we started thinking that there was probably more to the story. As I think about it each time, I think that I should have said something to Katie. I feel it is her responsibility to tell her own boyfriend as soon as she can that she slept with his friend when he was away. It just seems like a respectful thing to do. Jill, knowing what went on, seems to have done the friend thing and told Joe he truth. Every time I think about it, it switches. I also strongly feel that if Katie didn't want the secret to get out like it did, she shouldn't have cheated and she should have told Joe herself. 

I want other people's opinions. Was it wrong of Jill to rat Katie out? Or was it ok to tell Joe since Katie was keeping it a secret? Is it worth it to get Jill's side of the story??