Ben Sheffner (Copyrights & Campaigns) has the story, with all the links you can want. Patrick Frey confirms that the commenter does indeed seem to be the lawyer (something that's generally worth checking on the Internet). Here's an excerpt from Sheffner's long and detailed post:

Patrick Frey is a Los Angeles County Deputy District Attorney. He also has a blog called "Patterico's Pontifications," which features news, analysis, and commentary on subjects including politics, the criminal justice system, and media bias. Frey's blog includes a disclaimer on the front page making clear that his blogging is completely separate from his work as a prosecutor ....

Frey recently wrote a long and detailed post about allegations that 2 prosecution experts in a Louisiana murder case manufactured evidence that helped send the defendant, Jimmie Duncan, to death row. Frey's post included extensive analysis of the forensic evidence, and the results of an interview he conducted with a prosecution expert witness (who was not accused of manufacturing evidence)....

Duncan's appellate attorney[,] ... Kathy Kelly of the Capital Post-Conviction Project of Louisiana left the following comment on Frey's blog:

I'm am [sic] Mr. Duncan's lawyer. This case is currently on appeal. You are not the prosecutor, the judge or a forensic expert. You have noted contacting several people who are potential witnesses in the case and who will be called as witnesses later on in an evidentiary hearing. As a lawyer you should no [sic] that you have no business talking to witnesses when you are not a party to this case. Cease immediately or I will file an ethics complaint with your state bar.... You are a memeber [sic] of the general public you have no right to be demanding that this child's autopsy or medical records be turned over to you. Again you are neither the DA or the JUdge [sic] in this case.

What? It's an ethical violation for an attorney who blogs as a hobby to act exactly as a reporter: interviewing witnesses and seeking relevant documents? On a case in which he has no official involvement as an attorney? ...

Sheffner analyzes the California Rules of Professional Conduct, and concludes there's nothing at all unethical in Frey's opining on the case. I'm not expert enough on the Rules to verify this, but I can say that First Amendment law should and likely does protect Frey's speech even if Sheffner is mistaken and the Rules do indeed purport to restrict Frey here (which I have no reason to believe is the case). The Court has upheld (in Gentile v. State Bar) some restrictions on lawyer speech about his client when there's an impending trial, chiefly because of concerns that the speech may unduly influence jurors. But this rationale certainly doesn't apply to speech by someone who isn't a lawyer for one of the parties, and who is speaking while the case is on appeal (and thus before judges, who presumably know enough not to be unduly influenced by public comments about the case).

Frey's response, which is much to his credit: "I don't intend to cease exercising my First Amendment right to speak about matters of legitimate public concern because some lawyer who is mis[re]presenting the content of my post threatens a bogus complaint to my State Bar."

UPDATE: Prof. David Hricik (Legal Ethics Forum) agrees with Sheffner.

Patterico, the blogger involved, quotes the message from the lawyer.