bad injectors will usually smoke white at idle but it sounds like that could be his problem. 01-03 dmax's are notorious for bad injectors thats why GM extended the warranty on them to 7 years or 200K. its pretty unlikely but hopefully your buddies is still within that. its from when the truck was first put into service so if his truck wasnt bought untill late 02 then he might have a chance. more than likely though hes out of warranty. injectors are not cheap so tell him to be prepared. a full replacement at the dealer can run $4k+ just to give you an idea because the injectors are buried within the heads which makes it a lot of work to replace and is generally thought to be why they go bad because of rapidly changing temps. the amount of miles on them doesnt matter. some go within 30K and some last 200K. im at 130K with stock injectors, guess ive just been lucky so far. on later models they moved the injectors to the outside of the heads and they dont seem to have the same problems. so i would tell him to get it to the dealer and have the balance rates checked. that should give an indication of whats going on.