Eww Bad News Help me!!!!!?

Okay I knew this boy every since I was little I just met this girl a couple yrs ago and she started dated him They being together for almost 2yrs and he like a brother to me and he always here for me when something happen he let me know when n he told me to keep a eye on her but anyways he got a new job and he be to busy for my best friend so lately she told me she be horny and since he always at work she don't know what to do and she don't want to cheat on him so my advice to her was get a vibrator and don't cheat on him but she don't want to seem desperate so one day she met a new guy then 2days later they had sex w/o a condom n she had his *** inside of her when he left my bro came in like 2 mins later and decided to eat her out and he ate the boys *** out of her I feel very sick as I tell this story but I feel sad n guilty because I know that happened and he is like my twin I feel that I should tell him that happened because i know he will do the same but I don't want to lose my best friend I just feel that im stabbing him in the back right now and if i tell him imma be stabbing her in the back Idk what to do but In my heart something tells me I should tell him n whoop her *** for doing that to my bro and and thats it going to be okay I would find a new n better friend what should I do Yahoo Follow my heart or leave it alone