Let him buy it, then remind him to clean his gun after its been fired, no matter how many rounds went through it, he'll beg for an Ak after that!
Seriously, I never clean my AR anymore because its so filthy, it doesn't matter if its 50 or 150 rounds, the thing is caked in carbon!!!!
Also, tell him the 7.62 penetrates better, has cheaper mags, is cheaper to shoot, and in a SHTF situation would be readily availible in Opfor ammo caches. (5.45 would fit the latter case best, but 7.62 would be found anyways) The only thing .223 has going for it is fragmentation, but thats only a %50 occurance, and only at certain ranges.
BTW, everything I have said is garnered form info found here, if some of you would like to clear up my facts, feel free to, for you probably know more than I do.