“Thou shalt not bear false witness against thy neighbor” (Exod. 20:13). By implication of this negative command it is forbidden to say a shameful thing about a neighbor not in his presence, i.e., that if he were to hear it, it would anger him.¹ As it is written, “Thou sittest and speakest against thy brother, thou slanderest thine own mother’s son” (Ps. 50:20). But if he has committed sins and has been reproved privately but he has not accepted (the reproofs) then it is permissible to reprimand him publicly for his evil doings, with or without his presence.²

footnotes: ¹ Arakhin 15b. ² Maimonides, Hilkhoth Deoth, 6:8.