girls what would you do if this happend to you!!

so im with this guy that i love and ive been with him for about 1 year and 3months but in the begining we started soo wrong everything was a dissaster!! but some how i still liked him and also back then i had mixed feelings about still liking my ex and it was all crazy! soo later on when me and my current bf started getting really closer too each other he tells me that his sisters friend went over there house to go stay over and that in the night the started to watch a movie in his room soo then he told me that his sis got sleepy and went back to her room and left the friend in his room since she was supposibly still watching it soo after she leaves the friend trys to hit on my bf and goes on top of him he says that he tried to get her off but he couldnt so she kept telling him things and he didnt wanna do anything well supposibly but that he then garbs her ass and tells her there now get off and so then he leaves too go shower like at 12 in the morning and he even put it on anything that nuthing else happend and that he regrets its because he started thinking about me but i dont know too this day it still gets me really mad i feel like if he cheated on me and i know he says he regrets it and that he wont ever do it again but i just still feel like somthing is wrong and i feel like adding the girl as a friend on myspace and asking her what happend soo can you give me any advice??