So I have a work partner (public safety). We spend 50+ hours per week together, so people in our line of work generally get pretty close. Furthermore I've known this guy for quite a few years, and generally thought he was a pretty cool dude.

I also use BlackDragon's non monogamous relationship model, so this is all in the context of that.

So anyway one of my plates and I recently had the talk, and she'd came around again afterward for another night of sex. He knew all of this, and knew her as well (since she's one I met through work). I'd been seeing her in an FB situation for about 6 months or so. No dates, no boyfriend type shit, just sex once a week (or a little more infrequently). It was time for the talk because despite her being covertly aware of my non-commitment, she was really pushing the issue.

So anyway the next week she was a little colder than usual, so I just left her go. I pitched a meet and she "had plans". The next day I get a text from her that specifically named of some of the other people I'd hooked up with. I immediately suspected him of being her "plans", so I called him and asked. He denied it up and down. So anyway I let her go, and figured she'd come back around eventually.

A couple of weeks go by, meanwhile I'm still seeing other FBs and gaming new women. A few days ago near the end of shift, he's like "I want to talk to you". He's pacing furiously, and clearly nervous/afraid looking, I already know what he's going to tell me. He drops that they're "dating", and said that he was the one that told her about my other specific women (which I didn't lie about, but never mentioned any names to her). He also said "I wanted to tell you to your face like a man, instead of over the phone when you called me", but that was 2 weeks ago.

So I'm not necessarily upset that shes seeing someone, I kind of expected that. Rather I almost feel like a total contempt for him. He lied to me, betrayed my confidence, and then LTRed one of my plates.

I'm not quite sure WHAT to make of it. Its not jealousy, but more like pity and/or dissapointment. I feel like this is almost going to be a LSNFTE, but could also potentially be her trying to get back at me. In either case pretty much all respect I had for the guy is shattered, and so is the dynamic of trust (which absolutely has to exist for work). I was really kind of caught off guard by this. While I wouldn't consider him a close "friend" our type of work dynamic is a bit different.

This isn't so much about HER, cause i'm reasonably sure she'll be back. If she's legitimately dating him, she's going to get bored as hell sooner rather than later. He also specifically said "she's not doing this to get back at your or make you jealous", which if she told him, is almost certainly ABSOLUTELY why she's doing it (remember look at what women do, not what they say).

What do you guys think about this? Overreacting on my part, normal/natural?

Edit/Update: So after reading through the replies and thinking about it some more. I'm currently reading Dale Carnegie, so trying my damnedest to not be critical! Guys, I appreciate all the feedback (positive and negative).

Regarding her: yeah no changes happening here (still going to stay NC with her). I have several dates lined up and other FBs. One of the things I didn't consider is how naive Billy is, and if I were to replate her later, he might lose his shit. So I'll cross that bridge when I get to it, but erring on probably not. If/when we cross paths I'll keep things polite and professional.

Regarding him: I even think my initial reaction was a little bit of a knee jerk. I think I probably blurred the lines between "work trust" and "personal trust". So as far as working with him goes... I'm just going to watch my own ass, and won't be doing any favors for him. I'm more valuable to the company, there's no need to jeopardize my own standing. I'm not going to fuck the guy over, but if the time comes where he needs a vouch/nod from me.... well he burned that. I'll be keeping him at arms length for sure.

I wouldn't say that we were "close friends" by any stretch, but I definitely expected better. I thought we were tight, this proves we weren't. He didn't seem like the typical AFC/billy beta so I figured we had that common ground. He talked a good game, and like he was a player too.

In any case I've already wasted too damn much time and attention on this nonsense. I got burned this time,and that's on me. Lesson learned though, choose my confidantes more carefully.

Now, back to having an awesome life. Thank you gents!