Hilchos Rechilus, chapter 9, section 13

Everything we said only applies if one knows that the cheated person will go to beis din and act according to Torah law. But if one knows he will do as he pleases by seizing the cheat’s property, returning the merchandise, or not paying him what is still owing without sanction of a beis din, one should not tell him. For to avoid rechilus requires three conditions:
a) The informer who tells the person he was cheated must know the story first hand and not from others.
b) There must be two informers.
c) That the cheat loses no more than what the Torah demands. If one knows that the cheated person will make him lose more, one may not tell him anything.
Together with the first five conditions there are now eight conditions, and it is rare for them all to be fulfilled. Even if they are fulfilled, the informers will be helping the cheated person do a transgression even if they are many, unless they testified in beis din and the beis din allowed him to seek redress as explained earlier in chapter 6 paragraphs 9 and 10. Therefore, one must be careful about this. See paragraph 6 earlier.
Now, my brother, see how many people stumble in this matter. When someone buys something and halachically acquires it by moving it (meshichah) and asks someone if it was worth the price, the person not only doesn’t praise it, but tells the buyer he was cheated without even knowing the market price, for the price can often change in a short time. One also needs to know if the amount he overpaid was enough to consider he was cheated according to Torah law and whether enough time passed for him to show it to a merchant or relative. Otherwise, there is no benefit in what one said. It only causes hatred and counts as rechilus.
Sometimes the person saying the seller overpriced merely hated the seller and the article was sold for the correct price. Often, the person incites the buyer to return the item by force or send it back with someone else, or not to pay (which is often against halachah and tantamount to robbery). When he brings the merchandise back and the seller refuses to take it back saying he does not have to, this leads to great quarrels and hatred.
See how many evils the rochel did: He transgressed, Do not go as a rachil among your nation (if he is not careful of what we wrote above) and, Do not put a stumbling block before the blind by advising to return the merchandise contrary to Torah law or causing other losses. He causes controversy and transgresses the negative command, Do not cause distress (lo sonu) one to the other, and many other negative commands that result from controversy. Therefore, one should be very careful not to get involved in such matters unless he carefully ponders all we have written above.