In certain situations, one may — and sometimes must — provide people with negative information that is important for them to know, either to avoid being harmed or for some other constructive purpose.

Whenever faced with such a situation, one must explain why he is saying something that would ordinarily be prohibited, so that the listener understands that the prohibitions regarding loshon hara are not be transgressed. To say anything that the listener thinks is loshon hara when, in fact, it is not, constitutes speaking avak loshon hara.

Sefer Chofetz Chaim, 7 Cheshvan, page 122

dyjs