Some advice please...?

We if she is ur best friend & u tell her she may not belive u and she may think that u are tryin to break her and her boyfriend up, so its reallly your choice because if u keep it to urself maybe it was just a one time thing for her boyfrined maybe he won't do it agian, but if he does and u don't feel comfatable with it then u need to tell ur bestie maybe she can work it out with him or sumthing , but as i said before its ur choice....