The first time, when he realized that Naku wasn't guilty, he didn't need a single proof. Inspite of all the letters Kala tried to poison Dutta's mind with, he just believed that Naku had no part in Supriya's elopement. He believed in her innocence because his heart told him so. He didn't need anyone to prove it.
This time around, inspite of being mean to her, never once has he accused her of being unfaithful. He is angry with her, no doubt, but his anger is justified. She lied to him, and he cannot forgive her. But the fact that he didn't believe Suresh's story both times(am hoping he won't believe him on Monday) isn't that amazing! I mean there is proof in front of him and yet he trusts his wife. Now we know why Nakusha thinks he's a good human. Also, it must be the power of her love, else why would a cynic like Dutta defend his love? This alone has me floored. When you love someone this deeply, all you need is love, and faith. In Dutta's words, aisa pyaar bahut mushkil se milta hai.
I have high hopes about Monday. May the optimist in me win.