Hilchos Lashon Hara chapter 10, section 1

If someone saw someone harming a person by stealing from him or damaging his property, whether the robbed or harmed person knows it or not, or that he shamed him or distressed him or embarrassed him. If one clearly knows that he did not return the stolen item and did not pay him his damage and did not ask forgiveness for his wrongdoing, even one saw this oneself, one may relate the incident to people in order to assist the injured party and to disparage evil deeds before people. But one must be careful that seven rules are fulfilled.