Your friends HE351ve sliding vanes might be sticking... the early 6. 7 vgt turbos were notorious for sooting up and sticking open or closed or part way in between... mostly something to do with the software, which can be updated by the dealer.
Likely it is ALSO time to clean the sliding mechanism and turbo exhaust housing... IIRC,there is a procedure that can be done on the truck, but also IIRC, driving the truck like he stole it once or twice (think high rpms and high egts) helps burn the soot off and might be a temp solution just to see if soot IS causing the his problem.