Can a person register a conference using someone else's credit card and still get IEEE membership discount?
Generally speaking,


the name of the person registering does not have to be the same as the name on the credit card. You can pay for your co-author's registration if you want.
the IEEE discount applies if the person registering is an IEEE member, regardless of who is paying.


If your co-author is registering to attend the conference, it's fine for him to use your credit card (if that's what you want) and he can still get the IEEE discount, since he's an IEEE member.

If you're the one registering to attend the conference, you can't get the discount if you're not an IEEE member.