The Chofetz Chaim writes in his Sefer Chofetz Chaim Day 119 that there are 5 things to consider before someone tells someone else that they can get it cheaper by someone else. These 5 things are only if the storekeeper is looking to cheat the customer( meaning he/she knows that one can get the item at substantially reduced price elsewhere), then and only then would a person be allowed to tell another that they should shop elsewhere. The 5 things are,
1)You must be certain that your information about the store keeper is correct.
2)You may not exaggerate the storekeepers faults.
3)Your intent must be L’TOELES, for a constructive purpose and not because you happen to dislike this particular storekeeper.
4)You must be certain that there is no way to convince the person to avoid this purchase without telling them the faults of the storekeeper.
5)You will not cause the storekeeper a loss which is not permitted by Halacha. If your warning the person will result in a major scandal which will force the storekeeper to leave town or close his business, then you must remain silent.
If all these conditions are not met then one would be guilty of speaking Rechilus.
The Chofetz Chaim adds that a competent Halachic authority should be consulted regarding how best to prevent further fraud.
I see that your not looking for an Halachic answer, but, it happens to be a Halachic matter and not only a Mentchlichkeit matter. Sorry.
edited