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All he has to do is add your name as an AU only to report on your CR. He doesn't need to get or give you a card to do that.
It will have 0 impact on his score and or credit.
It will help you unless he lets those high balances report to the CRAs. I find it unlikely since he has an 812.
If he messed up and got a late payment it would not be up to the CRAs to fix. You would need to be removed as AU by him calling and removing you.
If he adds you as AU you get instant everything of the CC. History, payment history, CL, balance, etc.
You would need to report his death to the CCC and let them know you are AU and want to be removed.