No one is blaming Riddhima because she stuck with her marriage and moved on with Sid. She IS at fault because she won't let Armaan move on. :| She always gives him lectures about how he should move on in life like she did, but when he does begin to do that, she stops him. She interferes in his life SO MUCH. How is he supposed to forget her if she comes to him every other day with a lecture, if she butts into his life and tells him what to do, what not to do, who is right for you and who is wrong for you. She is constantly making things more painful and hard for Armaan.
If Riddhima ignored Armaan or didn't involve herself with him as much and kept her distance from him... then I would understand that Riddhima wouldn't be at fault. But she is always poking herself into his life and matters. She moved on with Sid, great. She should live her life with her husband now. Give Armaan time and his own space and he will gradually move on... but if she is going to keep on interfering in his life and keep on making him live his life according to HER decisions like she IS doing now, then she will obviously be blamed.