https://oag.ca.gov/firearms/ab991 (didn't get all the formatting on copy)
Pursuant to Penal Code sections 17000 and 27560, any person who moves into California with a firearm is considered a "Personal Firearm Importer" and is required by California law to do one of the following within 60 days:
Complete and submit a New Resident Report of Firearm Ownership (BOF 4010A), pdf along with $19.00 to the California Department of Justice, Bureau of Firearms;
Sell or transfer the firearm to a California licensed firearms dealer or to another individual using a California licensed firearms dealer to conduct the transaction; or
Sell or transfer the firearm to a California police or sheriff's department. Persons choosing this option should contact the law enforcement agency for instructions prior to transporting the firearm to the agency.
However, looking a little farther, Section 27570 talks to violations if the lack of reporting the importation only comes to the attention of the state after the grace period through submittal of a report. I'd suggest checking with a lawyer familiar with Ca firearms laws. Section 17000 says "b (2) In the case of a member of the Armed Forces of the United States, residency shall be deemed to be established when the individual was discharged from active service in this state."
Might be worth discussing with the local JAG office. I sort of think his returning to Ca with it would not be an issue but I'm not a lawyer.