No, No, No.

Hobbyists must have the right to question other hobbyists regarding their reviews.

The provider can be upset about it but you know.....

LMAO.  What do you think they are?  Priests?

Seriously. if posting a review for the world to see doesn't violate whatever confidentiality you think that you have, then nothing does.

I see this (client to client) as no different than when a lady asks for another provider's reference during screening, or the local "ladies only" board, among any other networking that goes on behind our back. I know I've been the subject of conversation; when a lady I've never seen before knows exactly what and/or how I like to spend our time together. This was information only my ATF would have known, and she of course was the reference used.
As long as it does not go too far, such as provider references being "yes I met him, he is ok,I would see him again" or, clients giving tips or recommendations like "she provides a quality GFE service", nothing too specific.
Another thought, this type of back channel/personal message/ email, can be essential info with regard to bad clients, ripoffs, cash n dash type of warnings. After all, we must all "take care of ourself, ... and each other". (oh no, did I really just qoute Jerry Springer?)

First, I think the men think we ladies communicate about clients amongst ourselves much, MUCH more than ever actually happens. When it comes to conversations about the clients, it's very tight-lipped (overall) among the providers. You would truly be amazed.
Granted, I can think of a client that is 'shared' between a circle of local ladies and we've discussed him, but, not in the intimately detailed sense you might presume. The conversations are very low-key with generalizations that are usually about personality, job or home situations, etc.  

Does it bother me that the guys backchannel each other about the ladies? Well, yes, and no. It would depend on what's being said, and the purpose of the conversation, and how it's being said. Are they respecting the privacy of our time together? Are they being thoughtful of my feelings about what's being said? Money should never be discussed. Period. Unlike what you are describing, I don't have anything to 'hide', or to worry about one client hearing something from another, so that's not a concern. Don't care. I have had clients call me or email me that So-n-So PM'd them some questions about me, and then I've asked what they were and he wouldn't tell because then he felt he was betraying the other guy. Ok, I can understand, but I just wanted to know the nature of the inquiries. If they were vulgar, or ungentlemanly at all, I would want to know, so that I may decide not to see him. But, since I'm always stressing the point of 'doing your homework', then if men want to make inquiries about me, I don't mind at all! I welcome it!

Re:Client/Client Communication (Depends on the Agenda)

It bothers me when I find out that clients (lol, special friends) have asked or shared information about me only when I find that the questions or answers are potentially dangerous, not answered in an honest way, or answered with malicious intent.  

Sometimes I find that hobbyists have agendas of their own and don't always answer or share truthfully.  In the past I have had one guy tell another guy about my SO at the time.  He had actually done a background check on my SO and shared some hurtful information and the person he shared this with then started a rumor that I was abused blah blah blah blah.  Not at all true and very hurtful to my SO.  If my SO hadn't trusted me completly this could have damaged our relationship.  Not to mention that it made me look like a victim, an image I never ever want to project.  I don't believe this was done maliciously to hurt me but it was based on speculation and innuendo, not on fact and could have been cleared up simply by talking to me not to someone else.

Jeez, I'm talking too much today.