Ski-Doo's oil used to be made by Castrol but I don't believe that is the case anymore. I seem to recall it is now the same company that does the oil for Cat (and Polaris?).
In so far as oil requirements, the E-TEC engine uses very little oil and is a hotter running engine than a typical two-stroke. I had a lengthy chat with one of the head engineers a couple of years ago and, as one would expect, he elaborated on the E-TEC engine's special needs. He stated that Ski-Doo and Rotax spent a lot of money developing spacial oils for these engines. At the time (as the 600 E-TEC was just being introduced), they were pushing their then-new semi-synth, saying it was superior to their full synth. He did say that they had begun work on a new full synth that would be better yet.
Your safest bet may be to get the BRP oil as they call for, but buying in bulk by splitting with friends. There are certainly other oils that can do the job but I suspect anyone not using BRP oil (particularly during the warranty period) may have to fight for warranty claims.
Finally, the 800 E-TEC is a very nice engine package. It pulls like gangbusters, starts immediately, idles with a smoothness that rivals (indeed exceeds) four-strokes, is very economical and virtually smoke-free. The only issue I have heard related to this engine (actually, all 800s in the REV-X chassis) relates to belt durability issues (the problem apparently due to the engine flexing too much in the mounts) Tell him to keep a close eye on his engine alignment.