Blackguards can spread their own foul breed of "redemption" just like Paladins do-- and this girl probably has one hell of a rage burning in her-- just waiting for a chance to be let out. Sure, he could probably finish her off, or take whatever he wants from whatever she has left, but that'd be like a twentieth level Paladin helping a kitten out of a tree when he could be casting remove disease on an infirmary full of children.
Or, he could teach the poor Druid to become stronger-- to seize what she wants and to hell with anyone who wants to take what's hers. He can nurture her hatred and her pain, and show her that the only thing that allowed those orcs to hurt her was her own weakness, her own uncertainty, her own morality.
The only question is whether he should punish those orcs himself-- severely and with great malice-- as a present to her, or if he should wait until she's healed and watch as she turns nature's power into a weapon against the entire tribe, eradicating her tormenters, their friends, their families, their women and children...