Happened to this guy as well. There's definitely something screwy with the new card filters in Quest Mode.
The henchman card should be a mostly harmless addition to your character's deck. You won't be able to use it for anything, but you can head off to a scenario with a "Banish a card to close" location and ditch the Boa that way.
Ok, so I'm not the only one. Weird bug and banishing it before something goofy happens seems like a good idea. Thanks