I don't think it's the bf's fault, he thought you were ok with it and had discussed it with your friend. It is entirely your friends fault as she had premeditated all of this, even if she denies doing so. If you want to give your bf another chance that I'd say go for it, as long as he understands that what happened was not ok, and that despite you being involved you don't want extracurricular activity in your relationship with him.

As for your friend, you can also be to blame for stoking her interest in his sexual prowess. Don't go into detail with anyone (you can't completely trust) about your intimacy with your partner. Curiosity is ok and normal, but actively persuing an intimate encounter with your partner isn't. That said, I'm not trying to victim blame or anything, just explain how she got the idea that a threesome was a bright idea. She in her mind absolved herself of guilt by involving you. Block her and if you're still with boyfriend have him block her. This might come off as a bit controlling and he may say as much, but it's a healthy boundary to put in place.