You bring up two issues: trust and care. And I disagree with your stance on both.
First of all, the man is irrevocably in love with Zoya. I think most of us would consider that a fact at this point. Are you saying that the man who slept on the floor because Zoya was sleeping on the floor doesn't care enough? Or the man who left Zoya after finding her just because he knew she would be hurt doesn't care enough? Or the man who admitted to having suicidal thoughts doesn't care enough? He cares, very deeply. And the question here is love versus responsibility. It never was. He loves her and that love should never be compared to anything else. He did choose responsibility but he would never let the love leave his heart. He is the same man, raised without his father's love. How do you expect him to deprive his own child (in his eyes, of course) of the same love he craved for all those years?
Secondly, the trust issue that you raised is also invalid at this point. Sure, he didn't trust Zoya before. But I think their relationship, post separation, has progressed to the point where he trusts her. Had Zoya told him about Tanveer before the bitch played the pregnancy card, I strongly feel that he would have trusted her. He just doesn't have any reason to doubt his best friend. He grew up with her. He has known her since their childhood. To be honest, I would trust my childhood friend too.