It definitely isn't just their fault.
Ayaan asked Zoya to carry on with the marriage temporarily.
Asad approved on Zoya's behalf, without her consent, plus it wasn't his place to consent.
And Zoya stupidly didn't even really reject the idea and instead did as she was told.
While most people blame Ayaan the most, I blame Asad and Zoya the most.
Ayaan was desperate to save his sisters life, so in his situation I think many of us would've begged anyone for help. Well I would, call me selfish or whatever, but I would do anything to save my family.
Asad however, didn't even think twice about the feelings of the woman who he claims is 'more precious' to him than his life. Of course he too like Ayaan would think about the life of his sister, but Zoya was his fiance so he should've thought about her too.
Zoya should've done what she does best, RUN! I don't get the girl. When it comes to other people's problems she's always looking for a solution but when it came to herself she just stood there and signed on the papers. Anyone of us would've run. But of course she's in love with Asad, so I guess she was willing to sacrifice herself for him.
I also mainly blame Asad for Zoya's pain, simply because he was the one in a relationship with Zoya, not Ayaan, so he should've thought about her. Ayaan is practically nothing to Zoya, so maybe she could forgive him, but how do you forgive someone who is the closest to you for deserting you like that?
If Ayaan's request was horrible, then Asad fulfilling that request is just as, if not more, horrible.