They never said being overweight is healthy, so this isn't really a counterargument. They're saying losing weight != healthy, which isn't the same thing. Take

They never said being overweight is healthy, so this isn't really a counterargument. They're saying losing weight != healthy, which isn't the same thing. Take anorexia or bulimia for example. You could be overweight, but that doesn't mean losing weight from an eating disorder is healthy. Same is true for a diet. Just because you're losing weight from a diet, doesn't mean the diet is healthy for you.