There is a saying by Stephen King...
The trust of the innocent is the liar's most useful tool.
And this is what Tanveer is using to her best advantage...Asad;s trust on her...
If Zoya wants to win this battle, she has to beat Tanu at her own game. Weaken the trust Asad has on Tanu. She knows this can never be achieved by confronting Asad or trying to tell him the truth. Nor can it be achieved by fighting with Tanu. Tanu is a disgusting woman who can go to any lengths. But, she has an egoistic confidence that Zoya will run away out of fear.. or will lose.
And, Zoya must use this confidence of Tanu against her. By letting her feel she is winning, and then giving a neat, shocking blow...