The Ruling on Informing People About the Drunkard
Question :
Is it permissible to inform people about a relative or a friend who does forbidden things, such as drinking alcohol for example, after advising him repeatedly? Or would that be considered disgracing him, even though one who keeps silent about the truth is a dumb devil?
Answer:
The obligation of the Muslim to his brother, if he sees him doing something forbidden, is to advise him and to warn him against persisting in acts of disobedience to Allah, Most High, and to make the punishment of sins and their evil effect upon the heart, the soul and the limbs, and upon the individual and the community clear to him. Perhaps through repeated advice, he may refrain and sober up.
If this has no effect on him, then he (i.e., the advisor) must take the shortest route to rid him of these sins, whether by informing the authorities, or by informing another person who is highly respected by this wrongdoer (such as an Imam or Islamic scholar etc.). In any case, he must take the shortest route by which the objective is attained, even if it means reporting him to the authorities, so that they may prevent him.
Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 6 Page 204