I just watched vide and I think Abhi is definately not the rapist .His reaction and expressions to Nahar Saloni's dialogues was "sridhar should be with pia. If he would have been with her this would not have happened to pia. Then walking out angry and upset and his body language.
Sridhar too was angry and little mellow after he heard about pia's rape. That still proves he cares for her. he even looked shock and upset after seeing pia's condition and when she huged him intially. Then he changed. his tone.
I think sri loves pia great deal and may be feels pia does not deserve some one like him. I may be wrong here but from his body language. I felt he is very much hurt but thinks this way he can give her freedom from all the miseries she is going thru right now.
That puts the prime suspect Ranjeet as pia's rapist.