It is a 30 dice check every time. I know that you can have the option to "trust her" which is ambiguously framed as "staying silent". It's no wonder that so many players fail to consider this option (especially on a first playthrough). In every other situation you are expected to micromanage the actions and major choices of your companions. It's a cheap trick the game throws at you, but it is beside the point here. While your approval rating with Shadowheart matters when she makes her "free choice", it is not linked to the romance. The romance does NOT matter even during her personal questline. It's a separate lane of events. There are no gameplay consequences to romances outside these special interactions.
I combined the Karlach romance with the Durge playthrough on my quest for the platinum trophy. Her reaction to his "boyfriend" turning into a demon was absolutely nothing.
