So if we the player didn't know ahead of time my response would be "what the hell are you doing with that abusive predator? Girl, your employer is bad. Get out of that job."
Okay. Good. This is now getting somewhere. So you say this to her, and then what? What might her response be, do you think? A "nice girl/decent person" response, or something else entirely? I have my own answer. What is yours?
This being a CRPG, I expect it to be somewhat accelerated as compared to a novel or long-run TV series (which themselves are rather accelerated to real life). My expectation would be a sort of "what do you know?" sort of response which would be an echo of things people say in real life situations as to why they stay in abusive situations. She's had a little bit of this here and there already. Her dialogue is full of risk assessment and a cynical attitude toward people in general. There's next to no intentional malevolence going on and in fact she is supportive of being respectful of people and following through on promises and the like.
Ideally stages should be:
encounter and work with due to mutual problems/survival > the discovery she serves Shar shadowed by the other major problem > trying to get her to move away from Shar > her shifting
And that's really skimming it. Ideally I'd work that story arc to be a character-focused novel or series. But this is a game and it'll cover the shift in about the same length as a law-and-order episode.
But I don't expect too much depth from games like this. They cast too wide a net to reach much depth.
That said, I support the idea of worshippers not necessarily sharing their gods' alignments. That's so very realistic and presents a lot of interesting story options from homicidal/inquisitional priests of Lathander all the way to adherents of Bane that are just don't have much experience with other gods or their worshippers.
