I think after seeing her interact with Ansh in that rain sequence, he accepted her as a part of his life but only for his daughter's sake, and with that acceptance came a level of respect which only grew once he saw the way she dealt with Payal and Palak. He found a wonderful mother in her, someone who would finally give his daughters the love and care of the mother that they deserve.
He has no bitterness or resentment towards her, that we see from Aarti's side, the only thing for him is that he can't give her the real place of his wife in his life because he is still extremely attached to Arpita at the emotional level. He is not falling for her, but rather I think he is silently observing her to see how she reacts to certain things, and making his own assessments about her which so far have been extremely positive! And I mean when two people are so unwilling to talk to each other, these looks and expressions are the only things that give away what they're thinking and feeling about each other!