Got a friend who's about to get his first AR, but he wants it in 6.8spc. Am I right in that you use a standard AR lower, but with different magazines? Or am I tottaly screwed up on this?

You are correct just install upper and use correct magazines and you are GTG.

only difference is magazine, barrel, and bolt carrier group.

I think only the bolt is different, not the entire carrier. I`m probably wrong but I thought I`d through it out there.

Bolt only.

These members have you on the right track. The only thing I would add is there are some 6.8 barrel configurations that are much better than others. Read up on the 6.8 some and you'll see that for the best performance you want:

SPC II (aka SAAMI II) chamber, or DMR chamber - These 6.8 chambers really lower pressure significantly over the restrictive older SAAMI chamber. That translates to increased velocities at safe pressure levels with some of the higher performance factory ammunition out there like SSA's famous Tactical Loads. Lower pressures are also much easier on your bolt.

1:11 or 1:12 barrel twist tends to work better with the 6.8 than the 1:10. You definitely do not want "9.5" twist. That is overkill and can lead to over pressure as well.
1:10 with a SPC II chamber works okay, and is the "minimum" required for SSA's Tactical Loads, however the industry is moving to the slower twists and you have plenty of solid choices available in the preferred specs. AR15Performance, Bison Armory, Noveske, Stag, RRA, Armalite (their new upcoming SPC II, 1:11 barrels), White Oak Armament, JTAC Supply's new WOA/Shilen 6.8 barrel, Denny's GTS, POF, Addax Tactical are some good examples of 6.8 uppers and barrels.  

There's a KISS chart posted on 68forums.com that goes over the 6.8 barrels of the major manufacturers. Feel free to visit the forum.