Legal Question in Criminal Law in California

No one uses a public defender in California unless they intend to plead guilty, so if he didn't want to plead guilty he should have hired a private lawyer. Maybe he's lying to you about how innocent he is.

Fund an attorney in filing an appeal.

That is, if it is still timely, and if he has any credible and legally admissible proof to support his claims to you, like a doctor's certificate and testimony that he was mentally impaired and incompetent at the time. He'd have to prove the attorney 'knew' he was 'incompetent', not just 'troubled', before you'd have a valid claim of ineffective assistance of counsel. Many, if not most, criminal defendants and convicts have 'mental problems' of some level and kind, that is what got them into trouble and prison. Also, most convicts claim they are 'innocent' and 'railroaded'. Judge the credibility of your nephew's claims on the case evidence, which had to be substantial enough to convince the police and the DA to file and pursue charges, and to think there was enough evidence to convict. They don't file for the thrill of it or waste time on 'loser' cases. Nor does the court accept a plea without being convinced there is supporting evidence. His attorney obviously thought he could be convicted or he wouldn't have recommended a deal to avoid a full statutory sentence after a loss at trial.