If he was properly registered and his ID matches the info he was registered with, then all he needs is his ID.
If you are entering Epcot from the main entrance, there are two double booths just to the left of center marked "Will Call". Go to any of the available open windows there. Tell them that you weren't able to print your GAD voucher, and give them the ID. They will look it up.
I did this two weeks ago as I left my voucher in my room...