I had a good friendship with Shadowheart, without a romance, I left it up to her to decide, and Shadowheart herself threw away the spear and didn't kill Eileen. But why should a character's decisions depend solely on having a romance with them? I guess if having a romantic relationship with Shadowheart and supporting her aspirations to become a Dark Justiciar in every way possible, or, on the contrary, despite having an affair, rarely taking her along, not giving her Noblestalk, not supporting her morally, then she will want to go the way of Shar and kill the Selunite, and that would be perfectly logical and understandable. Just like a mere friend might have some influence on her that would cause her to choose a different path, it's normal and perfectly realistic.
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.
