He could have easily walked away or said no. It appeared consensual and no one was forced - neither Nandini nor Kunal. I am not saying that Nandini is not responsible, she is but so is he.
She needs counselling so bad, she is falling into the trap where she has no clue of reality. This was cheating from the moment Kunal had non platonic thoughts towards Nandini, she became a willing participant she minute she took his hand at the bus station and then its downhill from there.
I hold both of them equally responsible and Kunal more so because he knows Nandini's vulnerability and should have said no.