He said it himself: A summoner's contract is absolute. So, as the summoner, just order her to not participate. Boom, problem solved. Until he's dead, they can't do anything, and I have a suspicion that there're laws against harming humans over problems between demons, and/or laws against trying to interfere with a summoner's right to order their contracted demon. Probably that would extend to preventing them from leaving the demon realm until he rescinds that order, and if not, that bone probably would be enough to handle whatever problems arise.