By being nice to her when she is in your prison cell.
When you spar with her to gain her trust, and you gave her some clothes to wear, you're on her good path. But the good path isn't finished and no CG scene for it is in the game yet.
You probably haven't advanced the main story far enough yet. You need to learn who the leader of her group was (which you learn in Dorgania from Lucilla).
