If the specs are similar to what I see on the Toyota website for 2006, he's gonna be pretty close to max if he has a family he's bringing, gear, pets, full tank of fuel and a loaded 22' coach, etc. Any shortcomings will be amplified on hills and mountains. Glad you are selling him your weight distribution gear. I don't think the Class III hitch is enough without that gear. I think the gear makes it a Class IV, which is what he'll need for a 5500lb 22' (or at least that's what I think a 22' weighs).
If he's sticking to the flatland and it's just a few folks and not a lot of gear, he should be ok. The truck is rated for 6500lbs, has body on frame construction and has a 118.1" wheebase. Should be ok with a 22', but as I said before he's most likely gonna be very close to the threshold and may exceed it give a certain set of variables. I wouldn't go larger than 22'. I'd feel better with a 19' with his Sequoia, with family, gear, fuel, etc.