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.