I suppose this is happening with increased regularity—a former client unhappy with his lawyer takes his gripes to the Internet. And the Internet postings are mostly disparaging, taking aim lawyer’s competency or his fees. The question arises whether the lawyer may respond without violating his ethical duties of confidentiality or breaching the attorney–client privilege.
A Real Situation
The Los Angeles County Bar Association Professional Responsibility and Ethics Committee recently addressed this situation in a Formal Ethics Opinion. 
The precise issue was in what manner, if any, may an attorney publicly respond to disparaging public comments by former client. The underlying quiz in this situation was whether a lawyer could publicly respond to a former client’s Internet postings that he committed malpractice and overcharged the client.
The Ethics Opinion
The Ethics Committee correctly noted that, unless the former client consents to the lawyer’s response or waives the attorney–client privilege, the lawyer remains obligated to preserve the former client’s confidential and privileged information. Under California law, courts may not create exceptions to the State’s attorney–client privilege statute, and this statute does not “permit an attorney to defend himself or herself by disclosing confidences or privileged information.” In other words, there is no self-defense exception to California’s attorney–client privilege.
The Ethics Committee did not, however, ban the lawyer from making any response. The Committee opined that the lawyer may respond so long as the response does not disclose confidential or privileged information, does not injure the former client in a matter involving the former representation, and is “proportionate and restrained.”
The Ethics Opinion cite is Formal Op. No. 525: Ethical Duties of Lawyers in Connection with Adverse Comments Published by a Former Client. You may access the opinion here.