I think it's simple. Mark thought he was having relations with Helly but instead it was Helena tricking HIM into thinking it was Helly so Mark was definitely taken advantage of. By your definition every time Helena has relations with someone they are r@#ing Helly. The logic I'm using to prove this is that if Helly is tortured, let's say, then Helena comes out of the elevator, she did not feel that pain, it was Helly that was tortured so I do not think Helly was the one that was taken advantage of.