If it checks out, and looks nice, and runs and drives well, it's probly a good unit.
The workhorse chassis owners think it's the best chassis ever made, and they are easy to get service and parts for.
The biggest thing for any unit is that all systems work - you have to test out everything, that it has service records for the engine and drivetrain, and that the tires are relatively new. A set of new michelins runs about three grand. If the exterior is glossy and clean and it doesn't stink inside, it will more than likely serve well. That is if he wants a unit that big.
I would take a gander at the DOT specs for the coach, CCC in particular, make sure it isn't one of those over built to the chassis units with a thousand pounds of CCC. Winnebago was pretty good about building at coach on a chassis that would support it. And with good tankage and capacities.
If he's not an experienced RV'r, or RV expert, he should take one with him or have it professionally inspected so he doesn't get stuck with expensive repair bills.
IMO that's good miles for that vintage.
BTW, here's the factory brochure on that unit in PDF:
EDIT: Just browsed the brochure. One note, it's one of the ones with "basement air" - the AC unit is in the back under the floor instead of roof units. They are notoriously troublesome with a lot of owners just having roof air units installed instead. They can be expensive to have repaired if they are non-functioning, and in many cases breakdown again and again.
Also, of note, the chassis, tankage, appliances, virtually everything about that coach is the same as what I have in a 31-footer. So we are at opposite ends of the size spectrum for a given chassis. I enjoy very, very good driving performance, whereas the 39 footer is going to be pretty much maxed out on that chassis. BTW, it looks like they built those only on the workhorse chassis.
I would not buy that size unit on a gas chassis to do a LOT of driving. Couple hundred miles and park it he'll probably be happy. If he's going to live in it and cross-country travel I'd look at a similar sized diesel - the Journey or Meridian. Expect to pay a bit more and the cost a bit more to run, but drive much much better and get better fuel economy. The Chieftan will get about 7.5mpg - on a good day.