He doesn't know about tanveer being such a disgusting person. Even though Zoya tried warning him so many times she was not able to prove it to asad.
Asad is a very kind person and thought his friend was in trouble to he went to save her without thinking about himself or his wedding.
He was completely unconscious and drugged so he didn't know what was actually going on. He thought it was Zoya. And tanveer made it seem as if it was Zoya too. He didn't want to sleep with tanveer but he couldn't stop tanveer because he wasn't in his senses.
Asad isn't at fault. This is no less than rape and tanveer is the rapist. It's as if someone would blame the person being raped instead of the rapist.