He's not installing the barrel extension is he? If not, and he's installing a barrel that has a factory installed extension, headspacing isn't that big a deal. And a fired casing can tell you a lot but it never hurts to check it yourself though.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek