Funny, this seems just like that one trick question, where the only correct answer is "The attacker is to blame".
But wth, i'll play along.
You are not to blame. You are not in any way, responsible for the two "lady friends". So you, are completely out of equation.
The "lady friends" are also not responsible for what happened. They increased chance of it happening, and should probably receive some kind of warning, but are in no way responsible for what has happened. In short - their mistake is being careless... that's in most cases not "illegal" from what i know. Stupid, but not wrong.
And of course the third "hidden party" being the attacker, who has attacked. He/she is to blame.
ps Pointing out that the story doesnt seem real, means nothing about your trustworthiness - having hidden motives is also not illegal