Question: Can you speak lashon hara (slander) for a constructive purpose?
Answer: First of all, I commend you for this question! The very fact that you are asking it says that you take the laws of Lashon Hora very seriously, at a time when, unfortunately, many people are far too quick to find (or invent) leniencies for themselves.
When something must be said, it is not Lashon Hora — but it is Lashon Hora, and forbidden to be said, until Halacha specifies otherwise. You have to be really careful about this, because the requirements for this in Halacha are detailed and absolutely critical.
For example, you must know absolutely that the thing is true, you must harbor no personal animosity of bias against the person you are speaking about, and you must know that it will actually alert the person to a business or other risk. And these are not all the requirements!
Rabbi Azriel Schrieber