A "voluntary" admission is NOT a "5150." However, as part of the background in CA he will be required to undergo a psychological evaluation and a medical evaluation, and as part of that he will have to disclose the fact that he was a patient. Whether this will be a disqualification or not depends on the nature of his admission and whether it falls under one of the disqualifying sections. Note that the pre-employment background in CA CAN NOT include questions about health-related matters until after a conditional job offer is made to the applicant. At that time he is eligible for a mental and physical health screening where the information would be disclosed. Intentionally concealing it would be grounds for dismissal even if it was not discovered until years later.
If his admission was entirely voluntary and he was free to leave at any time, there is a good chance he will not be automatically disqualified. But, if he was committed for any period of time, then he is prohibited from firearms ownership or possession pursuant to both state and federal law.
The following is a summary of the prohibition in CA:
"Any person who is taken into custody as a danger to self or others under Welfare and Institutions Code section 5150, assessed under Welfare and Institutions Code section 5151, and admitted to a mental health facility under Welfare and Institutions Code sections 5151, 5152, or certified under Welfare and Institutions Code sections 5250, 5260, and 5270.15."

