Why?
Because Riddhima has crossed all limits of indecency - the conscience one. She said that Armaan was always lying to her. Okay if we consider it as true, we should also look at the circumstances in which Armaan 'lied'.
1. He lied during the bet with Anjie but that was because he was a new to the whole concept of love and did not know what to do with those feelings. Also, Anjie lied too and she was more at fault considering she put her sister's faith and innocence at line but ofcourse Riddhima did not care.
2. He lied during Riddhima's adoption track but that was because he had promised her father that he wont say anything. So in reality, he was keeping a promise to someone and also thinking about how it was not his place to tell her the truth anyway and how much hurt she would be with the truth. The real fault lies with Shashank, Padma and Nani for keeping this a secret but ofcourse Riddhima did not care.
3. He lied about the Muskaan fiasco if that can be called a lie because he did tell Riddhima that he was going to help a friend and thats what he was doing but in any case he did it out of friendship but Riddhima did not care.
4. He lied during the Pari track because he knew that Riddhima would take it in the wrong way and as it is nothing happened...he was just helping out a girl in need but Riddhima did not care.
Now coming to Riddhima, when did she EVER trust Armaan?
She had no trust, no faith, no understanding where and when Armaan was concerned. She was always doubting him, questioning him and his intentions.
A relationship needs two people to build it and if Riddhima's hands were always trembling, then obviously Armaan's attempts would shake! She never trusted him EVER and that is the reason he could not be crystal clear with her because whatever be the case, she would question him.
But obviously Riddhima doesnt care. Way to go, Mrs. Modi.👏