What should a person do to atone for writing a master's thesis on behalf of someone else in exchange for gifts and temptation, and must he submit the documents that prove its forgery to the regulatory authorities, or should he leave the matter to God?

Your friend has done well by repenting from preparing the Master's thesis for the female teaching assistant, because this is cheating and betrayal, and what is gained from it is unlawful (haram). Similarly, it is forbidden for a man to establish a relationship with a woman except within the framework of a legitimate marriage. A woman should not be addressed except out of necessity, and with adherence to the controls of Islamic law. The conditions for repentance are: abandoning the sin, regretting it, and resolving not to return to it. If the sin involves the rights of another human being, then the rights of the wronged parties must be redressed. We advise your friend to inform the officials about the truth of the matter and submit the documents to them, as this removes harm and falls under the general category of obligatory advice. The saying "Let the king be for the owner" has no basis in Islamic law; rather, advice, enjoining good, and forbidding evil are among the obligations of Islam. We advise your friend to perform many acts of righteousness and supererogatory prayers, for indeed, good deeds efface evil deeds.