not my area of expertise but in order to buy them a badge I believe you have to link as a friend to an account in the system in their name anyway.
I love the idea of giving a badge to a friend. Maybe the best way of doing it is to say "Hey surprised I'm going to buy you a badge" Card of some kind.
badge information is here https://www.gencon.com/attend/your_badge
It is important to remember that if you buy a badge and plan to pick it up at Will Call you will need to show valid government-issued photo ID that matches the name on the badge. Also you can not pick up badges for other people. They have to get them themselves.