<human>: Condense the text into a few key points

So about three weeks ago me[26m] and my ex[27f] went to this big camping festival(similar to Roskilde) although we went there together we didn't hang together much. One of my best friend and a co-worker 'Stan' was there as well. 'Stan' and my ex became good friends while we were together.

So the weekend passes and everybody had a great time and it wasn't until yesterday that 'Stan' confessed to me at work that he and my ex had, had a one night stand. And they are continuing there 'relationship' afterwards messaging each other on facebook and such. I became pretty upset/angry(to put it mildly) and haven't spoken much to 'Stan' while I'm sorting things out in my head and that has made the work environment really awkward for both of us.

I want my and my friend to remain that way and I'm also more angry at my ex than my friend(since it was her idea that "we should remain friends after our breakup). I haven't confronted my ex yet but that's on my to do list. Like i said I've been thinking about this without reaching a acceptable conclusion, so therefore I seek out to you for some more solutions

Now my question to /relationships is: what would you do in my situation. Am I overreacting about this? should i just allow them to start there fling or what?
<bot>: Ex slept with my friend/coworker and I'm feeling betrayed from both of them. Not sure what to do.