Well, I've been there, in his living room in California, about 5 years ago checking them out, which he'd bought a few years prior. He's an executive and very conscientious and law-abiding, not some scofflaw with nothing to lose.
No, I didn't check the laws, serial numbers, registration, etc. But your comment did make me do a quick search which turned up many references to "California-Legal" AR-15s.
The 1989 law was basically a list of products, so it was easily evaded by making similar products and slapping a different name on them. California tried to tighten up the law in 1999 by specifying features, which was then worked around by adjusting the features. The 2016 San Bernadino shooting provoked another tightening, and I presume he bought it before that went into effect.
Here's a key quote: "In 2016, California enacted a law to provide a statutory definition for the term “detachable magazine” to clarify that firearms outfitted with bullet buttons are restricted. People who lawfully obtained these types of guns before Jan. 1 2017 could retain them as long as they registered them with the California Department of Justice in time." [0]
The article found after a quick search indicated that he needn't have been so worried, as the manufacturers also found quick work-arounds to that set of restrictions. It also mentioned that there were something like 189,000 currently registered... so yes, obviously extremely possible to have one.
QED was nothing, except that you know almost nothing about the topic; perhaps consider refraining from posting stuff about which you are obviously ignorant and then doubling down. Sheesh
[0] https://www.sfchronicle.com/projects/2021/california-assault...