Loaner Backpack Suggestions

A Luxurylite pack is the most adjustable one I have seen, both for torso length and girth. That might fit the bill for you, though it is not the typical internal frame design. It is possible to strap a pack bag onto the frame, rather than using the cylinders it is designed to work with, but it takes a little imagination.

The older SMD models (Starlite, etc) had adjustable shoulder straps, but you still have to have the belt in the person's size.

One way around needing varying belts is to get/make a belt extender. You can get a smaller belt, and use an extender (simply a length of webbing with the appropriate connectors for your belt, you can even make it adjustable). I'd think the main negative (and it would seem to be a minor one) is that any hip belt pockets would be further back on the belt the larger you made a small belt, if that makes sense.

That's a pretty crazy height range.

REI gear sell, my buddy got a huge gregory backpack for $15 with a broken hipbelt. Gregory replaced the hipbelt free of charge. At that price you could buy 2.

With new gear prices, you might consider buying packs for yourself to cover different situations that would naturally overlap with your friends purposes. A small summit pack for yourself would likely be small/form fitting for that purpose, but would not be overbearing for a 5'2 person to carry if you were carrying the tent for them. A large expedition bag 1-2 week trips with a bear vault, would naturally be bigger, might be conducive for your 6'2" friends.