What is the ruling on preferring a person over Allah and obeying him in His disobedience, and how does the one who does so repent from that?

Prioritizing a person over the love of Allah is major shirk. The noble verse: (And among the people are those who take rivals to Allah, loving them as they should love Allah. But those who believe are stronger in love for Allah) indicates that whoever loves anyone other than Allah as they should love Allah has taken rivals.

As for obeying a person in disobedience to Allah and prioritizing it over obedience to Him, this is a practical deviation, not shirk that expels one from the faith. However, it indicates a weakness in the love of Allah in the heart. The sign of a servant's love for his Lord is prioritizing His beloved commands and following His Messenger, peace and blessings be upon him.

The love of Allah has degrees, and prioritizing one's desires and disobeying Allah does not mean the absence of love, but rather its deficiency. Repentance from this is by dedicating sincere love to Allah and striving for the means that lead to it.