Sefer Chofetz Chaim hilchos rechilus chapter 9 halacha 13

The conditions discussed in halacha 12 to be able to help someone out who was ripped off, either overcharged or given a lesser quality product then expected, can only be used if you know the buyer would not take action into his own hands, but rather would take the seller to court but if he is known to take matters into his own hands you can’t tell him he was ripped off unless you meet 3 more conditions:

1. You yourself saw the product he bought and know he paid too much for the quality of that item. It can’t be secondhand knowledge. That most likely means you must be an expert at making evaluations. This is because, just as a court won’t accept secondhand testimony, so to in this case where he will take matters into his own hand, you must treat it like a court case.

 2. You can’t tell him alone, it has to be told by two people, as witnesses since the court would not extract money from someone without two witnesses, unless it is well know what the value of the item is and the court would not need witnesses to prove that he was ripped off, then you can go over to him as an individual to inform him he was ripped off, since this would be no different than a court.

3. But this is impossible if you know he won’t do anything to rash beyond what the court would sentence, but if he will take action, for example, leave the item in the store and steal some money, or another item from that store, or do any other unlawful thing which is more than what a court would enforce to be done, then you can’t tell him he was ripped off.

 In any event the chances of meeting all the conditions to be able to talk are very slight, especially since there are times that you might speak up out of hatred for the company he bought from. But even if you meet all the conditions, that just means you don’t have a prohibition of lashon hara, but you are helping him to sin because he wants to take care of things on his own and not through the courts. Even if many people told him he’s in the right and the store is in the wrong but he still didn’t go through the courts to fix the situation.

There are many times unfortunately now a days where a person buys a product, let’s say for example he signs an agreement to install new windows into his home, and maybe will pay in increments. He then tells his friend all about it and his friend says owe you were ripped off the going rate is this much. But the reality is prices fluctuate and you have to make sure you know the latest value, and when he bought the items before you can say anything like that to him, because all you are doing otherwise is instilling hatred into the buyer towards the seller and the seller might not have done anything wrong. You can be giving him bad advice, starting and amplifying a big argument and transgressing many sins like lashon hara if you don’t meet the conditions, placing a stumbling block in front of the blind by advising him to unlawfully return the item or any other idea which would cause a loss to the seller (assuming the item doesn’t have a return policy). You will also be transgressing the sin of starting a fight and the ensuing sins that will follow as the fight potentially escalates. It’s better to not say a thing and keep to yourself unless you absolutely know and are confident you are doing the right thing, and with G-d’s help, everything works out without any problems.