I think I used the wrong words. Maybe "I don't think he'll forgive her" is too strong a statement. :p It's just that Nick won't take her side, at least not yet. Instead, he would use her against the enemy. While doing so, he won't notice how deep he'd fallen for her until he thought he was about to lose her. 

I think it'll be easier if she confessed, like she'll be a state witness or something. But how do you think she'll react when she finds out that he's been lying all along? Can he not reveal his real identity even after she told him that she's a spy?