What would you if your best friend is dating your boyfriend/girlfriend?

I had a story about this kind of situation before. it happend in my high school too. my g.f is my best friend's friend too. Three of us are very close becasue we went out togather many times. everytime i have a problem regarding my girlfriend, i will seek my best friend advice and opinion becasue he is the one i trust most. one day, my girlfriend handphone was left inside my car after i fetch her back home. i knew i should not check her handphone message becasue it is her freedom to meet friend out there. But i did too. i saw there are many SMS from my best friend that store inside her handphone inbox. i feel strange and open the SMS. Each message content many sensitive words like sweet hear, darling, i miss you, i love you and etc. At that time, i am damn fed up and angry with my girlfriend and my best friend. Then, i questioned my girlfriend about this, but she questioned me back why i checked her handphone without her promission? Finally, she admited she was dating with my best friend sometimes when i am busy. I didnt said too much about this and i straight away break up with her and i didnt take my best friend as my friend anymore. i will betrayed.Double of the betrayed feeling.

Nothing like that has ever happened to me. You don't deserve a bf like him and a best friend who will just betray you. But I'm sure your best friend didn't intentionally wanted to hurt you, I guess she just couldn't hide her feelings coz' feelings really cannot be stopped. But if ever that happens to me, of course it would be too painful for me. But I doubt he will do that to me coz' I trust him fully. My best friend wouldn't do that to me either.