Hilchos Rechilus, chapter 9, section 13

All that we said is only if he knows the buyer's nature: that if he finds out about the cheating, he will not act on his own, but will go to a din Torah (a Jewish court case). But if he knows that the buyer's nature is to take the law into his own hands — to seize the seller's things or not pay him the money he still owes — it is forbidden to tell him. To tell in that case, three more details are needed:

a. The people telling must know about the cheating themselves. If they heard it from others, they may not tell him.

b. There must be two people telling.

c. They must not cause more harm than would come out of a beis din (Jewish court). If they know the cheated buyer's nature, that he will make the seller lose more than he would lose in a beis din, it is forbidden to tell him.

Together with the first 5 conditions, there are 8 details in all, and it is rare for all of them to come together. And even if they all come together, he is saved only from the sin of forbidden speech, but he is still considered as helping a sin. By law, the listener may not do anything based on his words until he goes to a beis din. So one must be very careful not to tell things to a person who tends to take the law into his own hands.

See how many people stumble in this. A person buys something in a shop and shows it to his friend to hear his opinion: was it worth the price or not? Some people not only don't praise it, but say, "It wasn't worth the price, and he cheated you badly!" They don't check carefully whether, at the time of the sale, it really was cheating according to the market price. They also don't check whether the time has passed for showing it to a merchant or a relative — after that, by law, he can no longer cancel the sale. So he speaks with no benefit at all. He only puts hatred in the heart of the one who was cheated, and he is a real talebearer, carrying words from one to the other.

Many times he says the item was not worth the price, and in the end it turns out that it was worth it. Meanwhile the seller loses money, because he pushes the buyer to go and return the item to the seller and tell him to his face that it is not worth the money: "And if you are embarrassed, send someone else, and don't pay him the money you owe." And when the seller does not want to take back the goods, it leads to insults and fighting.

See how many wrongs he has done. He broke "Do not go as a talebearer among your people" and "Do not put a stumbling block before the blind," since he advised his fellow to return the goods against the law, to get into a quarrel, and to break the negative commandment "Do not wrong one another," and several more negative commandments that come from quarrels.

So it is very right not to get involved in such matters until he has thought carefully about all the details. Then Hashem will help him not to stumble.