Firsty, Ritwik is saying whatever is written in the script. If he had to be partial towards Himruta then he would have made them won Love Power. Whereas Himruta are the only couple who haven't won a single love power. (As far as I remember)
As for Himruta and Ritwik friendship, Its way beyond these "love challenges" and even way beyond this nach baliye.
Its just that Ritwik feels really comfortable with himruta.
And he had that attachment with this couple since begining because in his very first Nach Interview, Ritwik named Himruta as his fav couple because himruta are "Chilled out" and "Mastikhor"
Not only this in an interview when Karan Patel was asked to choose a dance partner among the contestants, he also chose Amruta ,reason"He would be the most comfortable dancing with her, she is bindaas, friendly and above that a fabulous dancer".