I think I answered my question here.
http://www.thedieselstop.com/forums/f29/alcoa-center-caps-201376/
Post #7 explains it. The cap goes on after the wheel is installed and then it is held in place by the cosmetic bolt caps.
Does he have any part numbers?
Though the opening is probably slightly different with factory alcoas. I'll have to see and measure them once I get them home. Does he have the caps that say 'alcoa' on them?