She didn't mean to infringe on any territory by spanking fairies, but from your reaction I'm guessing she'd be welcome in the Forbidden Forest. You see, she's a fallen fairy and feels like she has a score to settle with the so called 'good' fairies. Her main job now is a pain fairy who can amplify any pain felt with her magic, and since Mina is an ally of her ward then I'm sure he wouldn't care if she assisted Mina every once in a while