Trigger situation for this question: someone asked for a doctor recommendation on a WhatsApp group with 200+ members. When a particular doctor was mentioned, one of the members wrote he had a bad experience with this particular doctor and wasn't recommending it. That person could have chosen to interact 1-1 with the requester but decided to inform everyone to discourage them since he thinks this doctor made a professional mistake when treating him.
Is this lashon hara?
Without addressing this particular situation, since whatever happened happened, are there circumstances where it might be allowed to publicize a negative experience with a service provider to discourage other people from using him?
Does it make a difference if one tells the specific person who has a "need to know" vs. a broader group of people who might one day need this particular service provider?
And is there a need to have a factual basis confirming the poor experience? (e.g., an insurance claim that was paid, a second doctor that would have confirmed it?) or is it all in the eyes of the client?
Assume everyone is Jewish if that makes a difference. And I'm of course interested in halachic sources vs. only personal opinions.