Another non-expert opinion. My observations and thoughts... as I used to watch the ads for 4 wheel campers and check them out if within a reasonable distance from home...
Most truck/rig combinations will sell for less than the individual items. I see this frequently, as the buyer has to make a decision about whether they want or like the camper and the truck. Also, it seems that the sellers of these rigs usually don't want the hassle of having sold either the camper or the truck and then try to find a buyer for the other. The sellers also realize they are taking a discount for selling the combo as they see it as a cost/benefit decision (one sale, no removing the camper, potential storage of camper, etc).
That said, the potential buyer has to assess whether it meets their needs. I think most truck/camper combos come onto the radar screen of potential buyers because they are already looking for that particular camper (4wheel, Northstar, etc) and just happened to see the ad for the combined unit. Unless the truck is exactly what the buyer wants, then there's always a purchasing hesitation (maybe I'll get it if the price is right).
All that gets factored into the ultimate selling price of the rig. Many potential buyers of campers already have a truck, so I think there's a stronger market for camper-only. On the other hand, if the price is right, it's a quick way for the buyer to purchase a ready-to-use rig. There's no hassle for trying to mount the camper to your truck (will it fit correctly, or does the bottom need to be raised to get it higher than the bed?). And there's a cost of installing a camper on a rig (the tie-downs, installation fees, maybe stronger suspension, air bags, maybe new tires, etc).
One final note... as a truck owner for the past 30 years, I would never buy a rig with a 6-inch lift. Too many issues with drive train alignment, suspension, emergency handling characteristics, etc. I have found a one to three inch lift can be managed with the appropriate after-market parts. Anything beyond that is questionable at best.
So my quick thoughts on the particular rig you are looking at is to figure the price of the truck and camper separately and then discount the truck significantly. After-market lifts and add-ons hold very little value in the used truck market. It doesn't matter how much the original owner spent for them. The original owner has to figure that he will get about 10 cents for every dollar of add-on. However the base truck can be easily priced on Edmunds, Kelly, etc.
I know it's hard to go to a friend with a low offer and try not insult them. Perhaps the friend needs to try selling it himself first to get a reality check.