The reason for this is that God has a sense of humour, a very cruel and twisted sense of humour but a sense of humour none the less. Weight and gaining it seems to be determined by a number of different factors, genetics, metabolism, how much activity we do, our diets. How much weight does this person actually want to put on? If you go to one of those slimming sites like realslimmers.com, they'll have charts telling you how much a person should weigh according to their height, age and gender, they should also have charts telling how many calories you need to consume to maintain your weight, using that as a guideline you could add a certain number of extra calories to your daily diet. A pound in weight equates with 3500 calories so you could theoretically add 500 calories a day to your diet which should add up to a pound a week. Another solution could be weight training, muscle weighs more then fat after all. It would take longer then pigging out but at the end of the day you would also wind up with a fantastic body.