I thought I'd save them some money and let them borrow a couple pairs of mine that have integrated adjustable bindings so they could save money on rentals and put it toward renting high quality demo boots instead of cavernous rental fleet boots.
Is there some downside to learning on a more precise ski?
In my opinion, calisnow, yes, but probably not as much a downside as others might think--depending on what type of high-end skis you're talking about. I wouldn't put your friends on a GS race ski, but many other high-end skis that are reasonably short would work OK for learning. It might not be ideal, as softer learning skis are more forgiving and probably lighter and easier to learn to maneuver around on. But even high-performance race skis today are much softer and shorter than those of the past. Surprisingly, a good slalom ski that is not too aggressively tuned would not be too hard for most people to learn on.
But, ironically, those "high-quality demo boots" could be a bigger problem. High-performance boots need to be set up well and customized--the higher the performance level, the moreso. High performance boots tend to fit snugger, meaning that without customization, they can be quite uncomfortable. More importantly, snug, stiff boots affect stance, both laterally and fore-aft, much more than softer, looser-fitting, comfy rental boots. The snugger the fit, and the stiffer the boot, the more critical the alignment setup needs. They may not be as warm, either.
As lousy as typical rental shop boots may be, since they will not be custom-fitted or professionally set up and aligned, that "sloppiness" may be favorable to a high-performance boot that is set up wrong.
It also will not likely save you much money. Most rental shops offer a package price for skis, boots, and poles. They won't reduce the price much if you have your own skis or boots. And in some cases, they might even be reluctant to adjust your skis to their boots.
So all things considered, I'd suggest that going with a basic rental package from a reputable shop is still your best bet for at least the first day or two. If that turns them in to "skiers," my next suggestion would be to bite the bullet and buy
a pair of good, higher-performance boots and get them custom fitted and aligned
by a reputable boot fitter. All of my arguments against renting high-performance boots reverse when they buy them and get them properly set up.
And then you can save them some money by loaning your skis, if you think it's appropriate.
Remember that those first couple days are critical in a skier's career. They can create life-long devotees of the sport, or they can turn people off forever. Yes, lousy rental equipment is one of the frequent culprits when people don't have a great first-time experience. But starting them out on poorly-set-up high performance gear would be like trying to learn to drive in a race car with misaligned wheels, brakes that pull to the side, and a grabby, worn clutch ....