That is not how it happens in the game, based my current playthrough. It was not a large amount of approval with Shadowheart that was required for her to make a good decision. I'd not even describe it as friendship at this point, it was simply a non-antagonistic relationship. It is not realistic, because in the absence of uncovering her secret (that she is a cleric of Shar), there wasn't even a dialogue with her where she questions her faith.
For the story purposes in this game 40 approval (which is the threshold for Shadowheart to throw away the spear on her own) is when you basically become bffs with most of the companions. I suppose the threshold might be so low because it was balanced with multiplayer in mind and you are much more limited in the amount of time you can spend with each companion in your party there.
I wonder if it depends on something after all. With me, she always chooses the option to "let go" of her parents and go the way of Selune.
IIRC these are the conditions for her to save her parents:
- 50+ approval
- Visit the grave of her teacher with her
- Find her old graffiti
- Have a conversation that should trigger right after you had both of the above
- Select "Only you can decide what is right..." when the choice itself is presented (if you choose to stay silent she will still decide to let them go)
