As I understand it, adding your friend to your accounts as an authorized user may or may not affect his credit score. It depends on whether or not the financial institution decides to make reports for the authorized users. Many do, but not all. If they do, then, assuming that your friend no longer makes late payments, the late payments will age and he will have a new account on his report that shows on-time payments, which might help his score. Since you don't intend to actually let him charge anything or pay anything, this is simply a trick to try to artificially help his score. It may not actually help his score much, since it doesn't erase the late payments on his report (only time will do that).
Keep in mind that, unless your friend is your spouse, you are playing with fire. You need to make sure he never finds out the credit card number. If he does and decides to use it, you are legally responsible and he has no responsibility to pay.
To answer your question about his card: When you add him as an authorized user, they might issue him a card with a unique number, but it is tied to your account. Therefore, it doesn't matter whether you charge something on his card or yours; it goes to the same account and you get one bill. So when his card arrives, destroy it, and make sure he doesn't get the number.