In my opinion, i think that Richa must have told Thakral that Kunal tried to rape her as she wants to destroy Kunal, so that he cannot be with his new found love Siddhi.
Richa's ego cannot digest the fact that Kunal chose Siddhi over Richa and fell in love too. In all the old episodes if you watch carefully she would always boast around with Vineet that Kunal can never love anyone else but her only.
That night was a big slap on her face when Kunal rejected her outrightly and also broke off all relations with her. Now she is terribly hurt and as a wounded lady wants to take revenge.
She also said that 'main tumhe kisi aur ka nahi hone doongi'.
So with this allegation, richa thinks that Kunal will be behind bars and Siddhi will loose Kunal forever and her MISSION ACCOMPLISHED.
Note: As some of you are saying that she will take the case back and try to win Kunal's goodwill, I doubt because now Richa knows very well that Kunal will never lover her back and she for that matter does not want him as a friend but lover. She always wanted him to leave Siddhi & his family and be with her forever. This will not happen as now she knows that Kunal cares for her as a FRIEND ONLY.
Please do write your comments and let me know if I make any sense.
kathy