It is paramount he learn in detail about the function of the AR, the value of things such as the TDP, mil-spec, HPT, MPI, shot peen, different barrel steels and other materials used, etc. so that he can best maintain the AR platform as well as educate customers on the features of the rifles he sells. He obviously needs to understand basic stuff (free-floated barrels, optics mounting, maintenance, cleaning, etc.). He should try to stock rifles which exemplify various accessories (fore-ends, stocks, grips, lights, optics, etc.) Which brings up another point. He should try to stock these items, or at least be able to order them to his shop and sell them to customers. For instance, he should be able to at very least connect his customers with the various accessory manufacturers in the AR industry, so they can get the products via the internet, if he can't mail order it to his shop or stock it at his shop.
Now, this may all seem a little complicated. But setting up an area of his counter space for this shouldn't be hard. He should get in contact with the manufacturers, get catalogs, and put them out on his counter. Contact the big guys, like Troy Industries, Daniel Defense, LaRue Tactical, Noveske, Colt, Magpul, Surefire, and Midwest Industries. Those companies sell a very wide range of products, and most every half-decent AR out there is wearing components from one, if not many of them. I have Midwest industries, Troy, LaRue, Magpul and Surefire kit on my rifle. he ought to also know how to install whatever he's going to connect the customer with, which means perhaps before he starts selling AR's, he should try building one or two himself. Once he has really studied up on the subject, he should go into business selling AR's.
Also, to those who know one AR brand from the next, and know quality, a big "we sell LaRue Tactical - Daniel Defense - Bravo Company - Colt - Noveske - Knights Armament" sign will bring them in en mass. It is rare for a gun shop to stock these "high end" brands (I don't see how they're high-end, they just make good kit...I digress) and many guys swoon at the opportunity to handle this good kit. If he can sell the guns in question at a reasonable price, word will get out "what AR are you shooting" "Colt, got it for $975 at XYZ's place" "wow no way! Colt's for under a grand? I'm checking that out!" and if he's a good dude behind the counter, he'll have no problem moving serious product. He'll also really create some AR enthusiasts, and as their interest in the higher-end AR brands sparks, he'll be ready to roll them right out the door.
Just my mileage, YMMV. Personally, if a local shop was selling good gear for a good price, did good work, and stocked the components, optics and accessories I was looking for, I'd be broke and buried in AR's.