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.