I've been cheated on before, so I can understand where
Demonspike666 is coming from. Ava proved to be an untrustworthy partner as well as an unfaithful lover. Sure we all make mistakes, but she deliberately made moves that went against her relationship with MC many times before she even got to the point of taking Bryce upstairs. She proved that she was unable to handle a relationship and was unable to keep her promises whilst in said relationship. MC seemed absolutely enamored with her before she went against his wishes and cheated.
I don't think many people realistically in MC's position would take a chance after something like that unless they do not have much experience with relationships in general. It doesn't matter if she's changed or not, I personally would not care if I were the MC. She smoked, drank, grinded on a guy that was not her partner, took him upstairs and slept with him. All after she promised the MC. The relationship lasted 3 months, I do not think many people would care enough to fix things that early in. You can just leave due to not being too emotionally invested at that point. Perhaps it is just me that thinks that way? I know if I were in a 3 month relationship, and my supposed boyfriend did that, we'd be over instantly.
Edit: I also do not think it is normal for someone to want to befriend their cheater. Most people are either sad or angry with them. So I am not sure how you expected MC to automatically want to reconnect with her or hear her out after she did that to him. Seeing her on top of all the other problems that he is dealing with, probably contributed to his hostility at her on the roof.