<human>: Dear great people of Reddit,

So after several weeks of momentary lapses and minor slip-ups, I finally had enough information to confirm my suspicions that my Girlfriend[16] and one of my good (or so I thought) friends[17] had cheated on me. After confronting her she initially denied it, but eventually admitted to inviting him over for a sleep over, and that several sexual acts were preformed by each of them while he stayed for the night. Apparently they hooked up three more times after this. She wouldn't say much more than this, but I put together much of the details myself. After I received her confession I walked home in a daze unsure who to talk to. This happened on a night where we had plans to go out, but she bailed on me saying she had to visit her grandmother in the hospital. As for my friend he had started dating a girl who he said he was In 'love' with, just a week before he slept over with my Girlfriend. Me and him were great friends, I bought him tickets to the sold out, farewell Alexisonfire concert in our town, and I didn't charge him a dime, we sat next to each other in ELA class for an entire semester without a single disagreement. The major thing that hurts me is that both of them had the nerve to hang around me for almost three weeks before telling me a single thing, and acting like everything was normal.

So now Dear Reddit, what should I do? My Girlfriend has called and texted me several times but I don't have the guts to reply. What should I say to her, should I dump her? Unless my girlfriend told my friend, I am unsure if he is aware I know of their infidelity, what should I say when I confront him? And should I tell his girlfriend about this? I want to tell her, but at the same time I would feel guilty if I was the reason they break up, does she deserve to know the truth, or should she stay blissfully ignorant?

Can you condense the text into three sentences or less?
<bot>: Long time GF cheats on me with even longer friend, who also has a GF, help on what I should say, and to who.