Rabbi Daniel Eidensohn has just posted “Lashon harah is only prohibited when there is no benefit or to’eles” parts of which I have reproduced below. Go to his post for more on this issue.
The main Torah source prohibiting lashon harah is Vayikra (19:16): “Don’t spread gossip amongst your people. Don’t stand idly by the blood of your fellow. I am G‑d.”
Rav Ovadiah Yosef (Yechava Daas 4:60): … In fact this is the way to understand the verse regarding lashon harah. “Do not speak lashon harah but don’t stand idly by concerning the blood of your fellow.” Even though there is a prohibition of lashon harah, nevertheless the second clause of the verse tells you that it is conditional on this not causing harm. Therefore you are obligated to inform others regarding certain matters in order to them to guard against loss and danger. This is expressed in Nidah (61a) that even though it is prohibited to listen to lashon harah but you should protect yourself from the potential danger you hear about. The Rambam (Mitzva 297) says that protecting another’s money is also included in “don’t stand idly by concerning the blood of your fellow.” … Therefore even if there is only a financial loss, one should inform your fellow man in order that he can protect himself from those who want to harm him. And surely when there is a possible danger to an individual or a group….