Hilchos Rechilus, chapter 9, section 11

All this concerns a case where one wants to warn the person so that he should not cheat. But if a person already took merchandise from someone and knows that he cheated in price or other deficiencies, it depends. If Torah law give no redress, either because the overpricing was less than a sixth or because enough time passed for the buyer to show it to a merchant or relative, or some other reason, someone who tells the cheated person he was deceived certainly transgresses rechilus. Since Torah law gives no redress, it is simple rechilus because [like a rochel, trader] he carries words from here to there. This applies even if the cheated person asks his opinion. It applies even more if one sees that the cheated person, through one's story, will cause harm to the person who cheated him such as seizing something from him or not pay him money still owing. However, if Torah law favors the cheated person (or if he can withdraw from the deal or get the excess money back), and if the cheated person would not have agreed to the deal if he knew he was being cheated, one may tell him the plain truth so that he can redress his loss. But one must be careful of the following factors:

Hilchos Rechilus, chapter 9, section 12

One must not exaggerate the sin or the loss. b) One's principle intent must be to further the interests of truth and help the victim, but not to gloat over the cheat even if one is certain he cheated. Similarly, if one knows that the victim will not seek redress but simply be despondent and hate the cheat, one should not tell him. Even more, in a case where the cheated person cannot annul the sale, it is a mitzvah to praise the deal to him. This does not transgress the verse, Keep far from a false matter (Shemos 23:7). (As the sages say in Kesuvos 17a, If a person bought something inferior in the market… one should praise it before him). c) If one figures that that the cheat will return the excess if he is rebuked, one should rebuke him and not tell the cheated person. d) If one can settle the matter in some other way without speaking bad of the cheat, one should not speak about him. e) If the cheated person is the type of person who will tell the cheat who told him he was cheated, it needs further research to decide whether one may inform the cheated person what happened because one will cause him to speak rechilus. However, it seems that if one figures he will listen if one tells him not to reveal one's identity, one should do so.