Nick physically abuses Caitlyn by hitting her. This is what lands him in trouble with the law; it's why he ends up being ordered by a judge to attend anger management classes. The judge also tells Nick that he must write a weekly journal detailing his relationship with Caitlyn, from the time he first met her right up until the present day. It's during the writing of that journal that Nick comes to realize just how badly he's treated Caitlyn and how in doing so he's repeating the violence meted out to him over the years by his father.

Nick also subjects Caitlyn to verbal abuse. His particular specialty in this regard is a particularly childish and destructive passive-aggression which we see when he accuses Caitlyn of not caring about him because she wouldn't pick up her phone and answer his calls when she was in class. Nick has just received yet another savage beating from his old man and he's forced to take the day off school to hide the bruises. But instead of dealing with his problems he takes out his frustration and pent-up rage on Caitlyn, someone he's supposed to care about.