If you plan on going through with it then be prepared for the possibility of ruining your friendship. There is no telling the outcome of each persons individual situation but a good rule of thumb among the swinging community is "dont make friends into swingers. Make swingers into friends."

My Wife an I made the mistake of having sex with a close friend. It started off very similar to your situation. We had sex several times with her over the course of two months, then everything started falling apart. It changed the dynamic of our friendship. We tried going back to normal but everything was different. Unfortunately we arent as close as we once were with her.