Disclaimer... I am not a legal representative nor do I pretend to be one.
According to Federal law, a stripped AR-15 receiver (any receiver), qualifies as a firearm, but it does not qualify as an AW because it lacks any of the "prohibited features" (pistol grip, folding/collapsible stock, bayonet lug, threaded barrel/flash suppressor, grenade launcher).
However, there is a point of law called "constructive intent", meaning that if a pistol grip or collapsible stock or FS, etc was in the in the dwelling (even though the parts aren't assembled to the receiver), then it's assumed the person had the intent or capability to assemble an AW.
If no other "prohibited" AR parts were present, then I'd call BS on the intent purpose.
Then again, you're asking about CA law, so the rules can be completely different.
Your bud needs to lawyer-up with someone who is familiar with Federal and CA gun laws.