Over 100K on the truck, so no warranty to worry about. If it was a new one with 400 hp and 800 torque I would agree BUT...it's not. Now the older truck has more than enough power to take any hills and such now. Guages to boot so he knows what is going on too. Towing a big trailer requires lots of power on tap as to not be a burden on the roads.
His truck, his money, let him spend it HIS WAY.
I did the same thing with a 6.ohno and loved it while it lasted..but then I wasn't towing and was racing Mustangs and Corvettes with a 8000 lbs truck.