How should a girl act after seeing her grandfather (70 years old) in a compromising position with the maid? Is what she saw permissible, forbidden, or a result of magic? And to whom should she confide this dangerous secret?

Your grandfather's act is one of the most heinous sins. The fact that he is an elderly, married man only increases his sinfulness. Religious texts indicate that a sin becomes more grievous when committed by someone who has fewer motives for it. You must advise him and warn him of Allah, and convince him to dismiss the maid. The maid should also be advised and warned, and any private meeting between them must be prevented. Initially, you should cover for both your grandfather and the maid, giving them a chance to repent. If he does not desist, you may inform your grandmother or one of his children who can deter him.