I don't see how one client telling another client what they heard, were told, or believe could be a problem. It does not turn the client into an agent of the therapist. The therapist who quit or the practice could get annoyed, I suppose, -but so what? Just repeating what one was told is not illegal in any way nor is talking to another client who self disclosed they were a client.

It might not be a good thing emotionally if one believes they were told something different from another client. But two clients can talk about anything they wish.