Laws of Rechilus / 9: 13 (112)

13. One whose friend has already made a purchase in which he was cheated, and according to Torah law he is permitted to claim what is rightfully his, he is permitted to tell him only on condition that he will bring the claim before a rabbinical court, however, if he knows that he will take the law into his own hands, he is forbidden to tell him.

Therefore, one must be sure not to get involved in such situations, unless he is positive that all the required conditions, mentioned above, are met.