That would be a dealbreaker for her. Not morally, very little is a dealbreaker for Isabelle morally, but because knowing Cy has (or even had) that capacity for violence would scare her too much for her to be able to keep a relationship with him going. Isabelle would probably play nice for a little while while she tries to decide what to do, and then she'd ghost him and start taking strict measures to avoid seeing him around campus.