What is the legal Islamic advice for a young man regarding his father whom he saw committing indecency with the maid, what is the legal retribution for this act, and how can the fatwa be conveyed to the father indirectly?

The sin mentioned is heinous, and your father must repent of it. Your duty towards your father is twofold: The first is general, which is to invite him to adhere to God's religion and repent from sins gently, without referring to his specific sin, emulating Prophet Ibrahim (peace be upon him) in his call to his father. The second is specific to the sin, which is to strive to convince him to marry to fortify himself, to change the maid, or to prevent his seclusion with her, and to warn her. If these means do not work, then you must supplicate for him for righteousness and uprightness.