I Found Out My Girlfriend Had Sex With My Best Friend On The New Year Day. HELP

Please guys dont get it twisted. Some of the stories i decide to share are not directly happening to me oo. if any should be about me i will surely let you guys know. But now this happened to my friend, some one i knew back in the days that year. He actually sent me an email, after saying that my number wasnt reachable, i later called him and asked him whats up and he gave me a load down of every, me sef i just tire so i decided to hear your opinion on this. please guys help me give this guy an advice. I will just paste the mail below:

"Hello Mr Camdi, I have been trying to call you but ur number hasnt been connecting. Please something happened to me and its not been long i found out and guess what; I discoverd One of my closest pal whom i had lived with for over two years had sex with my best friend in his house. My friend use to have a party on the 1st of January yearly, after the watchnight service we go to his place to catch fun with other invited people. While the party was going on we decided to spice up
the party by doing this game we all know as devil's basket, the game fell on my girlfriends turn and a guy was asked to touch three parts in her body of which he touched her bossom, her behind and another part in my presence, my girlfriend got upset and walked into the house(the party was outside the house but in the compound), i was going to stop her and explain that it was only a game that she should come back, she made a scene and got me angry and i shouted on her. She ran in this time around crying and with that act she almost destroyed the mood of everyone at the party. I left her, continued my job at the party(DJ) and my friend went in (he's fond of trying to be a nice guy to her when i make her upset of sumtin) to talk to her or whatever of sort. It was from there it happened, i wouldnt have known if his sister hadn't caught them in the act and told me on 3rd of January. I asked both of them and they didnt deny it. I'm trying to break up with my girlfriend but she's crying all heaven and hell her saying it was little of my fault, that my friend makes her happy when i dont and gives her more attention than I do but that she loves me so much and could die if i leave her. Pls note: My close friend and my girlfriend lives with me which has been since february. So please guys what do i do about this situation?
"