Tara says, "Tara can never betray someone's trust nor trust someone and then go back on that trust".
In the six months she has lived with him, he has seen Tara's love and respect for his family. He has seen the girl brave all the odds against her, winning her husband's love. He is aware of how she fought for the lives of her husband and the witness who could have set him free. He has witnessed the girl's pain in seeing her newly wedded hubby having to return to jail for a crime he did not commit. If after all this he fails to trust her or stand by her in her hour of need he has not only failed to trust in her words but also failed to keep up his.
The only way I can reconcile to seeing JR not believing Tara is for him to think that Tara has become mentally unstable after being subjected to so many shocks in quick succession such as witnessing her hubby's ticket to freedom being killed in front of her, losing her husband so soon after their remarriage, seeing her own sister wed a man much older than her and become her badi sasuma and coming to know that her husband has been put in solitary confinement. I want him to sympathise with her, even if he doesn't believe her rather than think that she is conniving and malicious.