Hilchos Rechilus, chapter 9, section 10

Since it is very easy to stumble in the ban on lashon hara and rechilus, we will describe some cases and explain them at length, so that a wise reader can learn from them about similar cases.

But so that the reader does not get tired of the length, I will write one case here. With Hashem's help, more cases will come at the end of the book.

The case: a person sees someone going into a shop to buy goods. He knows the buyer is a simple, trusting person who does not notice people's tricks. And he knows the shop owner, whose only wish is to find a customer like that and cheat him — in the goods, or in measures and weights. In such a case he must tell the buyer about that shop and warn him not to go in — even if the buyer has already agreed with the shop owner to buy from him.

And certainly, if he clearly sees that someone wants to cheat his fellow — in the goods, in measures and weights, or in the price — he surely must tell him, so he does not fall into it. But he must be careful that none of the details mentioned above in section 2 are missing.