Sid might not want Rid to divorce him but he hasn't stopped her from leaving him. He is selfish, he wants Rid to himself like any normal person who loves his wife but not at the cost of making her miserable. She is free to go from Sid.
Shashank might advise Rid on what is right what is wrong, but he is a great Father. He wants his daughter to be happy. If she chose to leave Sid and run to Armaan Shashank might be unhappy but he won't put shackles on her legs.
Sid assumed the divorce is happening because of Armaan and Shashank also assumes the same. But Rids decision had nothing to do with Armaan. She signed the paper because she felt over and over again Sid was trying to throw her away. He was trying to decide the course of her life.
Rid is not tied by social obligations either. The samaj is not coming between AR either. If that was the case she would stick to her marriage. But she has proved time and time again she won't stay in the marriage just because of samaj.