The second category of constructive purpose is to help those who are being (or have been) adversely affected by the subject’s behavior.
If one has personally witnessed a Jew causing monetary damage to another, to bring about restitution is a constructive purpose and thus makes it permissible to relate what has transpired. After ascertaining that the facts are correct, and after having spoken with the perpetrator, one must determine what will result from relating the information and whether or not halachah justifies effecting that result.
In a civilized society, the straightforward way of dealing with crime is to inform the authorities. However, reporting one’s fellow Jew to secular officials may result in his facing punishment undeserved according to Torah. Imprisonment, for example, is not an acceptable means of punishment for monetary infractions, by Torah standards. Providing information that might lead to a Jew’s imprisonment could therefore constitute malshinus (the act of being a malshin, informer). Only if it is clear that an individual is dangerous and poses a threat to others can he be spoken against for the sake of protecting others from harm. Such matters are extremely sensitive and complex, and demand consultation with a competent halachic authority.
Sefer Chofetz Chaim, 3 Kislev, Page 174