Well, first, thanks for the shout out, but there are plenty of good guys here that could answer this type of question.
Second - the know-it-all: He's not of this forum, is he?
Okay, here is the deal. With an AR lower, it isn't really "chambered" for any type of cartridge, but it does have a maximum size magazine that it will accept. The lower is pretty much like your basic rifle stock in that reguard.
A lower that will accept a magazine for say, 5.56, will also accept a magazine for 7.62x39mm. It will also accept a magazine that will feed, say a 6.5 Grendel, or a 9mm pistol, or a .45 caliber pistol.
Now, all the important pieces are in the upper receiver. In the upper receiver you will have the barrel, which has the cut chamber, and the Bolt Carrier Group. Some of the Bolt Carrier groups can be swapped back and forth between calibers, but you MUST change the bolt head.
A 7.62x39mm "upper" is an entirely different "weapon" in aspect that the entire unit is built to chamber that specific cartridge. If the upper and lower receivers have the same Take Down and Pivot pins, you just change it out with your 5.56mm upper and you are off to the races.
HOWEVER: and this may make you feel a bit better about your friend with ALL the knowledge. If you have a 5.56mm, and you want to switch to say a .45 ACP or 9mm, you will have to add a magazine downsize unit that will allow the smaller mags to stay locked in place, & you will have to change the Buffer Spring and Buffer for the smaller caliber and less powder contained in it.
Hopefully that answers what you needed to know. If not, I am sure someone will be along shortly to assist with any holes.