He's grappling with the changes in her. She's becoming someone he never knew, and he finds it increasingly difficult to hate her. Even his own mother, who he thought would share his sentiments, seems to like her.
"No. Just because she lost her memory does not make her any different. She is still a Conrad, and her father even died because of her. That reason alone makes me unable to forgive her. " He was trying hard to remind himself of this.
But his eyes started remembering his mother's face earlier. Even after he told her who Ariana was, his mother was not bothered by that statement. In fact, she wants to have a mother-daughter relationship with the girl, which Zayn really can't understand.
Does his mother know something he doesn't know?
He knows very well that his mother is not the one who can lie. So, her happy face comes from her true feelings.