If he has an automatic he can install an exhaust brake that will lock up the torque converter during de-acceleration.....
Without an exhaust brake there is no effect in down shifting either an automatic or standard because of the way a diesel engine breaths during de-acceleration. .
An exhaust brake mentioned by most of you is the answer. . I'm currently sitting in an RV park in Yuma after close to 4000 miles on this trip towing my 5er... it weighs just a little over 14,000... I've been from Spokane, to Carson City, Las Vegas, Santa Fe, Albuquerque, now Yuma and will head back to Spokane later this week... I've been down to Laughlin, across the divide and at this point have not had to use the brakes descending hills with the exhaust brake ... and of course back to Vegas, Salt Lake, into Montana and finally Spokane...
All my trucks do very well towing with the PacBrake...