Buying a badge for another person at the door

Okay, here's another probably stupid question for you :)

There are quite a few people in my area going to Origins for Pokemon Nationals.  One of these people failed to pre-register himself and his son, and he's not getting there until very late Thursday night.  He's worried mainly about his son being unable to play Nationals, since that's basically the whole reason for the trip.

In the meantime, I am scheduled to arrive sometime on Wednesday.  He wants to give me the money for himself and his son and have me buy their badges and event tickets.

I don't know what the rules are for buying that stuff at the door (this is my first year at Origins), so I want to make sure this is even doable before I commit to doing it.  He's willing to provide me with any paperwork necessary to authorize it, if that's what it takes.

Please let me know if this is even workable, and if so, what I need to bring to make it happen.