Well, it doesn't have to be IMMEDIATELY after, but I don't see why he wouldn't be able to (it's his account, after all). But if that's the case, then just leave them on there. It couldn't hurt, and it's not like it's costing him anything... it might even come in handy should there ever be a new card that supercedes the Centurion.

Or maybe you didn't understand what I was saying. I'm suggesting that he adds someone that spends that much annually as an additional cardmember, not for him to be added to someone else's account.