As far as I'm concerned, the doctor-patient privilege exists to protect patients. So a doctor shouldn't be able to say just anything to a patient without fear of a "trap". The Toronto Star article even quotes Weinberg, a Toronto-area plastic surgeon and member of the three organizations that have complained about Yazdanfar to the College of Physicians, as saying
"A physician has to be able to speak honestly to their patients. As a patient, wouldn't you want to go to your doctor and ask for an honest opinion even if it isn't to the benefit of a company, institution or another doctor?"The thing is, a doctor IS protected when he speaks his honest opinion, even if it isn't to someone else's benefit. There are many justifications for slander; in many jurisdictions one simple defense is "truth", that is, what you are saying is true. In some places you can't reveal the truth if doing so does no good but only does harm, but a doctor advising a patient on issues pertaining to safety can certainly not be accused of gratuitous harm.
I support any efforts by doctors to uphold a code of ethics, but let's be sure your ethics are actually ethical and not just protectionist.