I am not a lawyer and this is not legal advice, but I'm pretty sure he has to keep them and you have to buy ten-rounders.
12020.b(19-32) does not include "family" as a legitimate reason to transfer high-capacity magazines. A friend of mine has an old semi-auto AK and tells me that the law is basically written so he can either take/sell it out of the state, destroy it, or be
buried with it. The DOJ even sent him a letter basically listing all the things he couldn't do with it, including
willing it to anyone in the state, even immediate family. I expect the high-capacity magazine law is written and interpreted in much the same way.
This said, b(22) says he can lend them to you at the range, while he's there with you. It's a small comfort at least.