If the dyno guy didn't pay attention, and the owner who was there watching what went on , paid attention, but didn't do anything...well a comedy of errors
Like 4Jaw said....Aluminum radiators are not happy at all after a severe overheat, especially if the motor blows a head gasket. Common problem.
The aluminum is thin, the tubes have larger flat sections than a regular brass radiator and when they see too high of a pressure the tubes get oval shaped, which crushes the fins together and it gets hard to get air through the fins to cool anything.
At freeway speeds the air pressure was probably enough to force air through, but at slower speeds the fan can't pull it as well...so it overheats.
Did the truck owner make the dyno operator note that the truck unnecessarily overheated on the dyno, but it had been fine before?
If you spent a ton of money on the truck to fix the problem, before the dyno guy had a chance to look at anything, or acknowledge the problem, it's now all on the owner.