Well, first of all, I would think she would be upset that you, broke the rule... you dated her best friend. I dont know how serious it is but, thats a problem. If you still like her, than you should explain it to her friend, and call her and tell her. I wouldnt expect to much, if she knows you dated her friend. Maybe she will be ok with it, but I doubt it. I dont know how old you all are, so the behaviour, I am just guessing at. All you can do is try. But try to put yourself in her shoes. How would you feel? Good Luck