There are multiple ways for Shadowheart to go during that scene, depending on how you have been interacting with her up to that point. I’m on a ‘good’ run, and I have maxed out her affinity. I didn’t have to roll any dice. I only had to do something that is pretty obvious, if the choice is there. I guess if you haven’t been going that route, convincing her not to kill said personage is quite hard.