I'm not sure how much your friend had his f350 cranked up, so power wise it should be similar, but their are so many factors such as gearing and transmission ratios as to whether or not it out pulls the Ford.
Having said that, The FL70 will last forever pulling that toy hauler, with the most important factor SAFTEY! The bigger tires, brakes and mass of the FL70 will stop so much better than the Ford. The question about brakes tells me he has all ready had a stop or 2 that he wishes not to repeat.
2001 GMC 6500 Topkick, 22' box, dropped frame, designed to fit into a 9' garage door. 3126 CAT 6spd Man Lo-Pro 19.5's w/ 3.07 rear axle ratio