He definitely can't receive the magazine. If he already owns a 30-round MAC-10 magazine, you can disassemble the magazine, remove one part (probably the baseplate or the follower), and sell it to him as a "rebuild kit", but some people are (understandably) skittish about that.
The RPB Industries Inc. or SWD Inc. MAC-10s and MAC-11s are explicitly listed as assault weapons (CA Penal Code 12276(a)(10)), and it is very bad juju to ship those into the state, except under some extremely limited circumstances. There is a not-inconsequential amount of paperwork involved in those, and a responsible buyer would probably have let you know about all of that beforehand.
In either case, even if it is not an RPB or SWD gun, if it is a rifle (has a shoulder stock), it may still fall under 12276.1(a)(3), "under 30 inches collapsed and capable of being fired in the shortest configuration". I'm not familiar with the gun in question, so I couldn't say. A fixed stock might not run into that problem.
In short, I am 99 and 44/100% sure that you should refuse to complete the transaction.
I am not a lawyer; this is not legal advice. It is worth exactly what you paid for it. Liability limited to 2¢, $0.02 USD, by mail, or one free sandwich if you should visit the Bay Area.