The blame lies 50-50 with the two fucktards and the attacker. In a perfect world, they wouldn't have to worry about being assaulted...but this isn't a perfect world. It's their responsibility to take the preferred course of action and their course of action placed them in harm's way. I suggest that you separate yourself from them, they don't deserve sympathy and they're using you (the guy that tried to prevent this from happening in the first place) to remove responsibility for their poor choice.
On a side note, it's a good thing you didn't stop them physically. That would have ended really badly for you.
On a side note, it's a good thing you didn't stop them physically. That would have ended really badly for you.