John,
I think the important word here is "knows".
I believe it is fair to say something if the person saying it does actually know first hand, and beyond any doubt that what they are saying is absolutely true.
Over many years working in the guiding business, I've heard numerous stories about unethical/dishonest/illegal actions of others in the industry, and many of those stories were based on nothing more than rumour and heresay.
They may be true, maybe not.
I personally know of an instance where one operator was/is spreading the word far and wide about some dishonest actions of a rival operator.
I also "know" as fact that the accused operator had absolutely nothing to do with it.
In that case it was nothing more than sour grapes and professional jealousy, but throw enough mud and some will eventually stick.

I guess I don't have a problem with disclosing information regarding a dodgey operator, publicly or privately, so long as that information is "known" to be correct.

Otherwise I think it is maybe best left unsaid.
Just my 2 cents.