SovereignAxe is correct about the short barreled rifle. If I may offer a bit more technical info concerning the AR.
Make sure the pistol has an actual "pistol" buffer tube. Putting a carbine buffer tube on the lower and then removing the "stock" part will provide "intent" to the prosecutor.
Also, the AR-15 is designed as a modular weapon such that the upper receiver will separate from the lower receiver rather easily via two take-down pins. Typically, uppers and lowers are interchangeable allowing one to buy a lower and then have several uppers of various calibers/barrel lengths which may be mated to the lower to best fit an application. Because of this, it is VERY easy for one to become a felon by simply(accidentally) mating a pistol upper(barrel less than 16 inches) to a rifle lower. Which bring me to this...
Any rifle MUST be purchased AS A RIFLE and MUST REMAIN a RIFLE unless registered as an SBR etc... So... one cannot take ANY rifle lower, remove the stock and mate it to a pistol upper!!! This means that the AR pistol lower must be PURCHASED as a dedicated PISTOL lower. Usually, this is ensured by the manufacturer stamping the word PISTOL on the LOWER. Have your friend very this is the case before purchasing.