Will the questioner be held accountable as a "dayyuth" (cuckold) for being unable to prevent his mother from committingZina, even though he disapproves of her actions?

Zeal concerning Allah's sacred injunctions is a characteristic of the believer and indicates manliness. If the non-mahram man lives with you in a non-independent room, it is impermissible. So, advise him to fear Allah and threaten to inform your father. Accusations of adultery are a grave matter, especially for a mother, and Islamic law has strict requirements for its proof. If you find something suspicious regarding your mother, advise her gently and kindly, remind her of Allah, and threaten to inform your father if she does not desist. There is no blame if she becomes angry with you. If she does not respond to advice or threats, you must inform your father or someone among her relatives whom she respects, using allusions rather than explicit statements unless absolutely necessary, for the continuation of this relationship is forbidden by Islamic law. You must limit yourself to informing only those who can prevent the evil, as the original principle is concealment.