What is the ruling of Islamic law regarding the actions of a man who committed adultery, drank alcohol, used drugs, and participated in a shooting, all with the aim of exposing a criminal gang for the benefit of the police?

Your friend must offer sincere repentance for the major sins he has committed, because the end does not justify the means. Both the end and the means must be lawful, and the means have the same rulings as their objectives. The story of Marthad ibn Abi Marthad al-Ghanawi illustrates this point: he refused to commit zina with his mistress, 'Anaq, because Allah had forbidden it, even though he needed her help to smuggle prisoners. She exposed him, but Allah blinded his enemies to him until he succeeded in his mission. There is no harm in spying on thieves, highwaymen, criminals, and mischief-makers.