I have to admit,the only innocent in this situation is the woman that was cheated on. In the grand scheme of things, Claire, you got off easy. You already said that there was never a chance of you getting emotionally involved-- this was only someone to fuck. Imagine being this young woman-- naieve, trusting, and committed to a relationship she hopes will lead to marriage and she finds out that her intended is cheating on her. Her world is completely crushed. Completely. I feel terrible for her.
I agree that the biggest jerk in this situation is they guy-- and hopefully she'll get rid of him because he obviously doesn't treat sex and relationships the same way she does, and wants him to. He's a complete pig, in my opinion.
However, you're old enough to know better than to get involved in this situation. No, you don't have any allegience to her personally, but it's one of those situations where getting involved makes you culpable. Yes, he probably would have gone elsewhere, but that doesn't make it okay that you were the person he cheated with. If anything, stay out of this shit because someone willing to lie, cheat, and deceive isn't worth your time. Even in a fuckbuddy you deserve an honest, decent guy.
It's like the "Possible Desperate Housewife" situation thread. Most everyone that responded told the OP (who was thinking of banging his friend's married mom) not to do it. Because what happened to you is the outcome of situations like this all the time. It could have been a lot worse if she had really gone psycho. Seeing that her life was just turned upside down and fucked up, I can't blame her for going psycho. Still, the bulk, but not all, of the ire should be directed at the boyfriend.