Because it is so easy to err in the area of constructive speech, the Chofetz Chaim saw fit to offer the following illustration and expound upon it:
You are walking down a street in your city and you see Reuven about to enter a certain store. Reuven is a simpleton, a somewhat naïve fellow who is not alert to the schemes of crooked individuals. The storekeeper, on the other hand, is a shrewd fellow who has little trouble fooling people like Reuven with shoddy merchandise, inflated prices and dishonest weights.
In such a situation, says the Chofetz Chaim, you are obligated to warn Reuven not to enter the store. If he has already entered, advise him to leave as quickly as possible.
The same would apply if you see that Reuven has already agreed to a purchase in which he will be cheated. For instance, the storekeeper tells Reuven that a new jacket, which is on sale at a great price reduction, is a popular brand name. You happen to know that the item is actually a poor imitation of the brand name. Or, the storekeeper tells Reuven that the coat he is about to purchase is on sale for $400, but you know that the same coat is selling everywhere else for $315.
In such cases you must warn Reuven that he is about to be cheated. Of course, before doing so, you must fulfill the five requirements of rechilus l’toeles. Once again, these requirements are:
1. You must be certain that your information about the storekeeper is correct.
2. You may not exaggerate the storekeeper’s faults.
3. Your intent must be l’toeles, for a constructive purpose and not because you happen to dislike t his particular storekeeper.
4. You must be certain that there is no way to convince Reuven to avoid this purchase without telling him the faults of the storekeeper.
5. You will not cause the storekeeper a loss which is not permitted by Halachah (Torah law). If your warning Reuven will result in a major scandal which will force the storekeeper to leave town or close his business, then you must remain silent.
A competent halachic authority should be consulted regarding how best to prevent further fraud.