Is a gift to a driving instructor after passing the test considered bribery, and is a driving license obtained in this manner valid?

Bribery is forbidden by Islamic law. Allah has warned against it and condemned those who engage in it. The Messenger of Allah, peace and blessings be upon him, cursed the one who gives a bribe, the one who takes it, and the one who mediates between them. As for a bribe through which one obtains their right or repels injustice or harm, it is permissible according to the majority of scholars, and the sin falls upon the one who takes the bribe. In this case, what your friend did is considered bribery, and he must repent and not return to such actions.