You can, so long as he does not take that gun back into California with him:
18 USC 922(a)(3):
(3) for any person, other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector to transport into or receive in the State where he resides (or if the person is a corporation or other business entity, the State where it maintains a place of business) any firearm purchased or otherwise obtained by such person outside that State, except that this paragraph (A) shall not preclude any person who lawfully acquires a firearm by bequest or intestate succession in a State other than his State of residence from transporting the firearm into or receiving it in that State, if it is lawful for such person to purchase or possess such firearm in that State, (B) shall not apply to the transportation or receipt of a firearm obtained in conformity with subsection (b)(3) of this section, and (C) shall not apply to the transportation of any firearm acquired in any State prior to the effective date of this chapter;
The (b)(3) exception is for long guns purchased from FFL's.
BTW, a CA resident can possess all those California prohibited items outside the state of California.