Make sure he knows it would sound like a sewing machine on any and every grade if he wants to hold any speed.
I have two friends that had 18CK trailers (5,200lbs max) and got very tired of their 2007 and 2008 Tundra's down shifting on the slightest of grades. The trucks held speed just fine, but ran a LOT of rpms to do so.
I am sure this is how many gas rigs are, but no real experience with the others.
I find that hard to believe. They must be driving in OD if it was shifting a lot or had aftermarket exhausts if it was loud. I leave mine in 5th and it hums all day at 65mph in the low 2000rpm range with my 31' 7600lbs tt. If I hold speeds on some long grades it will downshift like any other vehicle, but it hardly sounds like a sewing machine. It's no low rpm diesel but it's also not loud or annoying at all.