I agree with Ranger: why tell someone who's intrigued by cycling that no, they ought to walk, or swim, or go to the gym instead? You have to like a type of exercise to keep doing it. We all know it feels great gliding along the road, out in the fresh air, going faster than you can walk! If all we wanted was the exercise we'd all be on exercyles. He oughta get ahold of a sturdy bike and try it, even if he goes very slowly at first. The Commuter forum had a great thread recently on bikes for people over 300lbs, including a description of a bike built for a 450lb person. http://www.bikeforums.net/showthread.php?t=54697
Of course diet, etc. is a big component of weight loss, but you have to take the first step--why shouldn't it be onto a pedal?