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: