Here's a thread with excellent pictures of an Arc 125.4 (four channel) amp installed in an '09 SE RG. Compare the size of the Arc (specs are on Arc's website) to the dimensions of your OEM HK amp. The comparison should answer the fitment question.
http://www.cvoharley.com/smf/index.php?topic=47523.0It will work, but unless your buddy has added a second set of speakers, the amp will work on two channels (the front speakers). I gave this amp and the wiring harness (from an '07 CVO Ultra) to a buddy with an '06 Ultra. It was a direct bolt on/plug n play installation and he's very pleased with it.
The amp is fine for the application (provided it fits and your buddy does some wiring mods) since it's the same amp HD sells with their "High Output Sound System," p/n 76418-06A, for all Touring models '06 and newer with a permanently mounted tour pak and 2 or 4 channel speakers. All your buddy is doing is mounting it inside the fairing.
EDIT: You are referring to the amp that is OEM on '06 and newer CVO Ultras?
Whether the loss of air flow to cool the amp by locating it inside the fairing will pose problems I have no answer.