When is it permissible to speak ill of a person?

Answer

It is not permissible to speak ill of a person. This is the rule of Allah Ta'ala. However, all rules have exceptions. The permissible exception in this case will not be said to be 'speaking ill'. It will be constrained by necessity – the need to impart benefit or lesson.

When a person sins flagrantly in the public, or when he advertises his sin as a virtue, or when he misleads and deceives people with his sins and haraam stunts, then it is permissible to warn people of his danger. Such warning will come within the scope of Amr Bil Ma'roof Nahy Anil Munkar which is an ibaadat of high merit.