My best friend is dating my boyfriend?

Question

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Well I moved out this past Saturday. I'm already hearing from anther one of my friends that my best friend and my boyfriend have been hanging out since I moved out. This friend have even told me that he caught them kissing one day when he thought I was still living there. Now keep in mind that i've been knowing my best friend for 8 years and if it's true I'm going to attack both of them. I look at it this way he'll rather pick a **** over our long friendship. I belive this friend because he's always been really truthful with me and always looked out for my best interest. What kind of trouble could I get into if I beat them?