Nuance,
I don't know your friends room, but he may benefit from duals as well. A pair of Power Sound Audio XS30's would be slightly over budget, and a pair of XV15's would be well under budget (your friend could probably get three). They use the CEA-2010 standard for their measurements, and my understanding is that they are pretty close to what Ricci has measured, so I think you could use that as a baseline to compare to other subs.
With a large room, there is some real benefit to multiple subs (trust me, I know). Yes, he would be giving up some output and extension vs the other subs (maybe), but he would get smoother bass response. I guess it comes down to whether or not your friend is a bass head or not and if he will really be pushing his system.
If he has any DIY skills, for $2K I suspect he can put the Funk Audio and JTR/Seaton subs to shame. The new Dayton 18" driver has been built and compared directly to a Submersive and it was pretty much an even fight. Difference being the Dayton driver is $250 a pop

$2K would easily put you in two dual opposed sealed enclosures using the Dayton drivers and powered with a pro amp. Should give you about twice the performance for right around $2K. Of course we are talking DIY....