Is reporting one's parents to the authorities for the crimes of "zina" (unlawful sexual intercourse) and incest considered disobedience to parents, especially given that the father had previously sexually assaulted the son?

Adultery is a grave and enormous sin, one of the major sins. Its occurrence between unmarriageable kin is even more forbidden, and whoever commits it with his mother is committing the major sins of adultery and disobedience to parents. Your friend must advise them and warn them of Allah's punishment. If they do not respond, he should threaten them with reporting to the authorities, but he should generally not report them due to the harm that would result. He must be careful not to spread the news lest he fall into the sin of false accusation. His parents' misbehavior does not nullify his duty of filial piety towards them. He must repent and become righteous. If there is a legitimate benefit in his staying with them, then he should stay; otherwise, he should seek a righteous environment.