First of all, don't his parents realise that their beta and bahu have a life of their own? I find Pradeep's parents, esp his father, claiming ownership over Mona as if she was their own unmarried daughter with no other commitments. Pradeep's mom should not have given that dumb advice to Mona about using the artificial coin...and Mona should have used her own judgement instead of being guided by hers. Also, the way his dad called Mona to his place over the issue with Taani...hello, she is your son's WIFE, he has a right to her time and the right to decide whether she should stay with him or rush over to your place. Even Mona did not think it necessary to consult with Pradeep whether she should go to her in-laws place at that time of the night.
Secondly, Pradeep's father is very dominating. He thinks he has the right to decide his children's life. We have seen how he was forcing Taani to marry Madhav. Even now, his parents have not understood that a girl may want some freedom before settling down. Pradeep did the right thing by standing up for his sister at the Holi party. It was something his father and elder bro should have done; but they were more concerned about their business deal than about Taani's self-respect.
Also, his father got Pradeep married at an early age without giving him a chance to complete his studies, which affected his entire career. All these frustrations have been building up in Pradeep for sometime, and that is why he feels closer to Mona's parents (who have more sense and know where to draw the line ) than his own. Even that relationship nearly got spoiled because of the coin incident, which took place because of his mom's stupid suggestion of giving an artificial coin.
Mona is also partly to blame since she gives more importance to her in-laws' opinion than to her husband's. Every man wants to be the hero of his wife; Mona may feel like that for him but she doesn't show it enough. I don't mean by romantic gestures; just by respecting his judgment occasionally. But I can understand her position since she can't offend her in-laws either.
Given these circumstances--a man feeling down in life and career, neglected by his parents and not getting enough time with his wife--it's not too surprising that a woman who pampers him and recognises his ability should be able to draw him to her. Again, i'm NOT justifying what he did; I am just explaining how it happened.