Yeah I think it's like this. Phasma cares about her Stormtroopers (she knows every single one's designation) albeit within the stringent restrictions of the First Order, and Finn was her favorite. I think if she defected to the Resistance she would take responsibility for her actions and I think over time Finn could forgive her or at least accept her. I'm all about Stormtroopers getting second chances and the idea of a Phasma redemption arc really fascinates me.