Is it okay if your guy goes out with her girl bestfriend?

Well believe me it can be a risk, I had a great best friend, she was like a sister and she didn't have a boyfriend so I used to bring her along with me and my boyfriend well something must have happened along the way, as after a party I had she had a little play around with my boyfriend and when I was talking to my then boyfriend about it we found she was playing two ends against the middle so to speak to try and get him...then went to where we both worked together and told everyone it was my fault as one is not supposed to take a girlfriend when going out with a boyfriend....and i would never have ever thought she would do such a thing, I trusted her 100%