What is the test for using a third party attorney in a medical malpractice case?

Defendants rely on O'Neil v. Cunningham (1981) 118 Cal.App.3d 466, 477, 173 Cal.Rptr. 422, in which the appellant, an anesthesiologist, was one of several defendants in a medical malpractice suit. The malpractice insurance carrier hired an attorney to represent it and the other defendants. In a letter written to his client-employer, the insurer, the attorney made false statements about the appellant. The letter came into the hands of appellant's superiors, who then terminated his contract. He sued the attorney for negligence.