<human>: Give me the gist of the text in a nutshell

I'll be brief as I she might see this  (long shot). We've been together for almost 3 years now. We're in a semi-long distance relation now about 200km apart. She was meant to see me tonight for a nice dinner with some people. I didn't feel 100% so I left the dinner and went to the gym to relax. People left my place going in various directons. She went to this event where she was invited by this guy. They sleep together. She wakes up the next morning, comes back to my place, sneaks in when I'm in the shower, grabs her stuff, leaves, goes back to her home 200km away. I'm confused about wtf is going on. Eventually she calls me later today and tells me that she's cheated on me. I'm gutted. I end the convo before I get angry and say rude hurtful things.

Turns out I know the guy and he knows me.
<bot>: 3 year long girlfriend with whom I was in a serious semi-long distance comes to visit me and bangs another guy. I know the guy, the guy knows me. What do?