There are medications that cause weight gain, usually through causing excessive hunger, slower metabolism, and excessive tiredness or sluggishness. I was on a medication notorious  for this and it was the only thing that brought me up to a healthy BMI after struggling with a particularly bad relapse of anorexia that lasted over a year. Not even close to 100 pounds of weight gain. So most likely did eat too much and not move enough, but the inability to feel satiated does not help that at all. Not defending the fatty because it's obviously a stupid excuse for huffing butter all day, but it's important to note that some meds can have side effects that correspond with weight gain.