Well. I guess the intended use of the AR is for defense. For when the **** hits the fan type of thing.
For a Colt 6920 configured for defensive use, just about all of the metal parts are quality. Not much to improve in that regard. Here are some other things he can add/change:
Mags- 10 or more Magpul PMags, or quality GI issue aluminum mags (Brownells mags are good too) with good followers such as the magpul followers.
Sling attachments and a quality 1 or 2 point sling such as the Magpul MS2 sling or the Viking MK2, among others.
He would do well to add an appropriate quality optic such as an EOTECH or Aimpoint CompM3, CompM3L, CompM4, or the smaller, lighter T-1 or H-1. I personally prefer anything with Aimpoint on it over anything with EOTECH on it. YMMV
I would personally change out the stock and the A2 grip for whatever his preference is. I like Magpul CTR stocks on rifles that are less front heavy, and Magpul ACS stocks on rifles that are more front heavy or in situations where better cheek weld is desirable. I think its kind of a toss up, so pick whichever one is your preference. VLTOR stocks are good too, but I like that the Magpul stocks I listed lock up tight. For the grip, 95% of shooters will hate the standard A2 grip. Probably will want to switch it out for something more comfortable such as a Magpul MOE or MIAD grip, or the ERGO grip. Hogue grips are good too.
If he wants to add a railed handguard, I would look first at the Daniel Defense Omega 7.0 rail. It is the easiest to install, requires no permanent modification to the rifle and is reasonable at about $250. Once he decides to add the rail then there needs to be some stuff mounted to it to justify it's existance. He'll probably want a foregrip, a light, and some rail covers to protect his investment. The new Magpul AFG (angled fore grip) is coming out in a couple weeks, looks pretty smart, and is only about $35. There is also the Tango Down foregrip, and several others. Lights are another one that can get expensive. Everybody raves about surefire x300 and x400. They should for the price. Streamlight has one similar that goes for about $175. Or he can get a Surefire G2 for $35 and a VLTOR mount for it for about $40. Whatever he wants to spend.
OR, he can just leave it the heck alone and shoot it just like it is, which might be a good idea for a while until he decides what he actually needs and likes. No need to rush out to buy a bunch of crap to put on a rifle that he's never even shot. But, you asked about good quality accessories, so there you go. Hope that helps.