Sefer Chofetz Chaim hilchos rechilus chapter 9 halachos 11, 12

 If one has already been overcharged or given an inferior quality product, the question of whether you can tell him about it really depends if he can do something about it. If he was charged over a a sixth of the going rate, then you can tell him and he can get his extra money back. If it was less than a sixth of the going rate, then we assume there is automatic forgiveness of being overcharged because it is within reason so you can’t tell him he was overcharged and it would constitute rechilus if you do. Even if he has not fully paid for what he bought, he has installments, and still can’t tell him, lest he does not pay the rest that he had originally agreed he owed. If it was an inferior or faulty product and he can still return it then you can tell him but if it is after the return policy then you can’t tell him and you should even praise the product if he asks you about it. That’s not transgressing lying because you are coming to avoid the prohibition of rechilus. This is based on a gemara I’m Kesubos 17a. If you know the person is the type of person that even if you tell him he was ripped off he won’t do anything about it, like he’ll be too lazy to return it or to call up and go through the hastle of getting some of his money back, and he’ll only have hatred towards the salesman for what happened then you can’t tell him either unless you know you can stop him from buying from there again. Of course, you have to meet the 5 conditions before being allowed to speak up:

1. You can’t exaggerate.

2. Your intent must be just to help your oppressed friend; therefore, it is highly likely that a competitor of the store he bought at can’t tell him that he was ripped off.

3. If you can rebuke the salesman and he will go over to the buyer and fix the problem then you should do that without telling the buyer.

4. If you can figure out a way to inform him of what happened without saying it straight out you should do so, for example if you can indirectly direct him to an advertisement, commercial, or article about the product he bought and he sees he was ripped off and will fix the situation on his own then you should do that and not tell him. We see that Hashem did a similar thing when he told Yehoshua to make a lottery to reveal that Achan was the one who was putting the entire Jewish nation in peril for the sin that he committed.

 5. If you see that the buyer by nature is a big talker and he will go over to the seller and name you as telling him that he was ripped off then it’s not so simple that you can tell him (and I am not sure if you would want to tell him) because you will be causing him to speak rechilus since he will tell the storekeeper who he got his information from. But if he promises not to drop any names then you can tell him.