I think they were both equally responsible, yet at times one or the other was more responsible for the continuation of the affair.
EJ didn't have to follow Abby to Horton Cabin and didn't have to respond to her flirting to him. Abby also could have stopped herself before it went any further, laughed it off with "Just kidding, I know we're just friends and I would never do that to Sami."
Abby didn't have to show up at the locker room or at the board room.
EJ didn't have to follow Abby to the supply closet or show up at the hospital when he knew she would be working there.
I've said it before in other threads: EJ has more experience with seduction, but Abby chose to sleep with him and to try to keep it going for a little bit afterwards. I think it became compulsive for her; I remember her asking herself, "What am I doing?" but that should have been a clue to get help rather than to let things just keep happening.