Hilchos Rechilus, chapter 9, section 7

The same applies to telling a person that someone else harmed him, robbed him, or did him some other wrong like this. It is not allowed until all the details above are met. Also, he must first rebuke the one who did it, and that person did not accept his words.

Without this, it is forbidden. One must be very careful not to decide for himself that it is allowed in these matters until he has thought carefully whether all the details above are met. If they are not met, he may break a full negative commandment of rechilus.