What is the ruling on dealing with a person who seeks assistance from jinn for good purposes, such as treating illnesses and undoing sorcery, and is his subjugation of jinn considered a miracle from Allah?

It is not permissible to seek help from the jinn, due to the corruptions it leads to, and due to the difficulty of judging them. It has not been proven that the Prophet, peace and blessings be upon him, or the early Muslims (Salaf) sought their help. A person may fall into charlatanism and sorcery under the pretext of seeking help from the jinn for good deeds. Ibn Taymiyyah mentioned that using them for permissible matters is allowed, but he warned against their negative consequences for those with little knowledge.

Karamat (Miracles performed by pious individuals): These are extraordinary occurrences that appear at the hand of a righteous servant. Belief in them is among the fundamental principles of Ahlus-Sunnah (definition) wal-Jama'ah. Many examples of them have been mentioned in the Qur’an and Sunnah, such as the story of the People of the Cave, Mary, the People of the Cave (again, possibly referring to a different instance or emphasizing), Jurayj, and Khubayb ibn Adiyy. It should be noted that a Muslim should strive for istiqamah (steadfastness/rectitude) not karamah. Extraordinary occurrences may appear to disbelievers as a form of enticement (istidraj). The proof of istiqamah is adherence to the Book of Allah and the Sunnah of His Messenger. Whoever claims that the subjugation of jinn is a karamah for him, his claim is incorrect; for karamah is a grace from Allah, and it does not come by seeking it. A person is judged by his istiqamah, not by his karamat.