The customer isn’t always right. But you must listen. Let’s face it. In the end it is the client’s money or business. I had a mediation where one of the parties was absolutely certain of how a trial would end. His attorney and I were fairly sure that his evaluation was not correct; that he well might well suffer an adverse result. So the litigator had to tread the fine line between (i) bringing light unto the darkness, seeming to be defeatist and possibly losing the client or (ii) supporting the illusion.
Make your point as clearly as possible, all the while being sure the client understands you believe in his position and will do the very best job possible. Emphasize that there is no certainty in court. And let the client know you are absolutely on his side. This is a situation where your mediator can be of material assistance.