If he has his heart set on an AR you can pick one up in .308, .243 Win, 7mm-08, or 338-08. if you want to spend the bucks you can get them in exotic calibers like .338 win mag, 338 lapua, 50 beowulf, 450 bushmaster 300 whisper et al. any of them would work for deer.
.223 CAN take deer but your freind better either be extremly picky about what shots he takes or handload something in the 90-95 grain range for knockdown power. 55 gr SP just aint gonna cut the mustard unless the critter is 25 or 50 yards away.
SW