9. If I have a large-capacity magazine, do I need to get rid of it?
No. Continued possession of large-capacity magazines (able to accept more than 10 rounds)
that you owned in California before January 1, 2000, is not prohibited. However as of January 1, 2000, it is illegal to buy, manufacture, import, keep for sale, expose for sale, give or lend any large-capacity magazine in California except by law enforcement agencies, California peace officers, or licensed dealers.
(PC Section 12020 (b)(19-29))
This was taken from the following California Attorney General web page of FAQ:
http://caag.state.ca.us/firearms/pubfaqs.htm
This FAQ answer does not mention the circumstances by which a hi-cap magazine can be loaned however (exceptions).
The loan of a lawfully possessed large-capacity magazine between two individuals is permitted if all of the following conditions are met:
(A) The person being loaned the large-capacity magazine is not prohibited by Section 12021, 12021.1, or 12101 of this code or Section 8100 or 8103 of the Welfare and Institutions Code from possessing firearms or ammunition.
(B) The loan of the large-capacity magazine occurs at a place or location where the possession of the large-capacity magazine is not otherwise prohibited and the person who lends the large-capacity magazine remains in the accessible vicinity of the person to whom the large-capacity magazine is loaned.
Interesting side note:
If the perps, as you mentioned, did not have the lawful owner present at the shooting location, then the loaner was in commission of a misdemeanor. However, I don't see that the perps have committed an actual crime, unless they bought the magazines instead of borrowing them. It's a loophole of sorts, that I do not readily understand why the morons in Sacramento left to be utilized. Maybe someone more hip on conspiracy law could tell me whether the loanees could be charged with conspiracy. Hmmm.
Look here for the complete regs on hi cap mags:
http://caag.state.ca.us/firearms/dwcl/12020.htm