The case is Bar Counsel v. Peter Farber, SJC 11171 (Massachusetts Supreme Judicial Court). This post is a repeat from a few years ago. I regard the opinion highly and think every lawyer should read it before taking any action adverse to an unhappy client.
Farber was the subject of an attorney disciplinary complaint. The complaining party, G. Russell Damon, testified against Farber. Damon accused Farber of wrongful conduct in connection with a real estate transaction. The disciplinary proceeding resulted in a reprimand against Farber.
Farber then brought a defamation case against Damon. Damon argued that he was immune because he testified under oath in a public proceeding. Damon took no other action to publicize his testimony.