Praise be to Allaah.
Warning others about a person who does evil and commits major sins is obligatory (waajib). If a Muslim fears that his brothers may be harmed by those people’s actions, he must advise them and tell them about these evildoers, so that no one will fall into their trap. It is also obligatory to advise the evildoers and those who commit major sins, so that perhaps Allaah may guide them. The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Religion is sincerity” They said, “To whom, O Messenger of Allaah?” He said, “To Allaah and to His Book, to His Messenger, and to the leaders of the Muslims and their common folk.”
(Narrated by Muslim from the hadeeth of Tameem ibn Aws al-Daari (may Allaah be pleased with him).