Unless he puts portable rollers under each rear wheel, all four if it's a 4x4, you do risk added damage to the transmission and possibly the rear end and even the engine. Pulling from the back is no different than pulling from the front if it's stuck in gear. It will either cause the rear wheels to skid or it will cause the engine to turn over, backwards. Not good. So my advice would be to make sure he brings some portable rollers and places them under the rear wheels, that way you know it won't damage the truck further.