The new M5 is awesome. The ZL1 is awesome. But they are 2 totally different cars in almost all ways possible.
So if that is the case I suggest your buddy go drive an Audi RS6 and report back. A shitton better than that M5 - well, IMHO
EDIT or an E63s for that matter.
EDIT EDIT - I say all this while spending some other rich dude's money
EDIT EDIT EDIT - ~11:30!