The stock OEM starter is the best choice IMHO. It's made by Nippon Denso, and the same basic starter is used by folks like Toyota and others in automotive applications.
BTW, why are you replacing the starter? Failures of the actual starter motor are pretty rare, and often what is really needed is to just replace the drive and overrunning clutch assembly. If that's all you need, you can save several hundred dollars just replacing the the bad part. What is it actually doing?
As for any special things your friend needs to know, the big one is that he has to drain the primary and remove the outer primary cover to gain access to the jackshaft bolt. The bolt, lockplate, and washer have to be removed to remove the pinion gear, before the starter can be slid out from the other side. The rest of it is pretty obvious and straightforward for a qualified mechanic. I strongly second fired00d's recommendation on the shop manual. If you don't want to buy one, see if you can borrow one from a friend or the public library perhaps. Any Big Twin manual from 1999 through 2006 should be sufficient to get the basic information and the torque specs needed to do the job.
Jerry