Jadis makes a compelling argument. If you didn't have the prior, specially acquired knowledge of the garden of Eden and Aslan being God then it prioritising your duty to your own mother by saving her life would be the morally right thing to do rather than upholding a promise to lion whom you do not owe the same allegiance to, especially since as Jadis rightly points out, Digory did not know the details of the promise when he was invited to agree to it.