It should bolt up to the engine, no problem. A new cross member will probably have to be made. The shifter will probably work. The hydraulic clutch system should work also, but I haven't looked at the 6spd too closely. The slave cylinder may be a little different. You may or may not be able to use your clutch, depending on the clutch splines, and if you have to use a SD clutch, you may have to use a SD flywheel also. I have never got past the thinking part on this.
My question is, why does he want to swap? If it's for "better mileage" or "higher gear" don't do it. Even though it is a 6spd, it does not have an "extra" over drive gear like some people think. The over drive ratios are almost identical.
If the ZF5 in the truck is bad, and he wants to replace it with the ZF6 then it would probably be worth the time, if he got a good deal on the 6.
I just don't want someone to do this and find out the ratios are close. If I ever run across a deal on a 6spd, I plan on doing the swap, because my 5spd is grinding.