ChronoSphere,
If you can shotgun the upper and lower open (still attached at the front pivot point), then this all that is needed to clean the rifle. After a few sessions of shooting, the upper/lower will loosen up enough to pull them apart.

Now on the other hand, if you mean that you can not even shotgun the back of the upper/lower open (with the front take down pin in the locked position), then you will need to make sure that the rear take down pin is as far outward as you can slide it to clear the rear lug. On the new build, you may have to give the pin a few love taps to make sure it has clear the rear lug*.  From there, it's as QUIB wrote, being that the RRA lug to rear receiver channel is very tight, and it may help to apply some lube and allow it to soak in before re-knee'g it.

* Do not go buck wild and try to punch the rear drift pin completely out of the lower receiver. It is retained in the receiver via a detent pin. With the rear take down pin all the way over, you should be able to look or use a tool to see/feel inside the receiver pivot channel to confirm that the rear take down pin has cleared the edge of the rear lug. By love tapping the pivot pin a little father out (read .005), you are settling the detent into the pivot pin end channel detent notch/ the receiver detent channel to allow the pivot pin to drift the needed amount to clear the upper rear lug.

Quoted: I am betting on the bolt being just inside the buffer tube. Take off the stock and buffer tube. CP

If that was the case, then there is just the option of pushing both the front and the rear take down pins fully unlocked, then holding the butt stock section and making the knee blow on the front hand guard.  This would cam the front of the upper off the lower and allows him to angle pull the upper forward/upward to clear such a bind.

Really, the whole game plan is for him to be able to just get the rifle clean to be shot the first time, without having to start wrenching it apart to do so.  Since we do not know his talent skill, if it does come down to having to pull the receiver extension off the new complete built rifle, we would be better suited suggesting that he take the rifle back to the shop/place of purchase to have it resolved.  Factory rifles have warranties (reason that you buy them built), and by wrenching it, he could void the warranty.


Almost forgot, ChronoSphere welcome to the site!!!!!

Quoted: Just for the sake of trying, push the pin back in and try pulling the bolt back and letting it snap forward on its own a few times, if the carrier is stuck back in the buffer tube maybe this will shake it loose...

Front pivot pin in/locked, rear pivot pin pulled out/ unlocked.
Pull the charging handle back, lock the bolt back on the bolt catch, then push the charging handle back forward and lock it into the upper receiver.
Now hit the bolt release, and let the B/C crash into the locked position.  
Do this a few times.

Now after the last time, push in on the Forward Assist and walk the carrier as far forward as you can.  Now holding pressure on the forward assist, then try again to shotgun the upper open over your knee.

Quoted: I pulled the bolt back, locked it, and released it (with the muzzle pointing down to let gravity help) several times and still not a budge of movement, and I've verified that the takedown pin is all the way out with the required click. I have a friend with a CAR stock wrench.. time to take it off?

Either take the rifle back to where you purchased it, or give RRA a call.
Wrenching on the rifle may void the warranty.

www.rockriverarms.com/index.cfm

Lube/grease on the rear upper lug is going to be your friend until the parts mate.