He's not going to change, it's what they said to him when he was a kid and what he said to his own kids.
I suggest that you tell your daughter that she needs to ignore him. Tell her she is built like XXxx. I am grossly overweight, my ex is chubby but over 6' tall, he never weighed more than 250 though and he's thinner now than he ever was. My daughter is tall like me ex's mom. She's built almost exactly like her except she got my boobs, my hair, and my eyelashes. When she stopped doing drugs she gained at least a hundred pounds. She is having a hard time getting her weight under control and she eats pretty healthy. She will have a couple of years of this according to what I have read then her body will straighten out and she'll be able to lose.
My granddaughter is tall and thin too. She does the normal thing kids do, she eats and eats and chubs out, she looks like she is getting fat for maybe 2 days. Then she nearly stops eating all together. She will wake up the next morning or so and she'll have grown an inch or two overnight.
I have had so many people say she needs to eat nothing but salads and veggies so she won't grow up to be like me....that's not only hurtful to her but to me too. We don't have a lot of money for groceries and often don't have money to buy fresh veggies and fruit.
If she's genetically programmed to be overweight then there's not a lot she can do about it except struggle and battle it the rest of her life. If she's programmed to be a more acceptable size then yes, she can eat unhealthy things and get fat, but chances are she will not do that if she eats normal foods as a child.
I see so many people who get away from home for the first time in college or when they get their first apartment. They binge and eat every unhealthy food day after day after day...they can't get enough candy, sugar, pop, coffee, pie, cake, etc....because they weren't allowed to eat it as kids.
Eating a balanced diet is good for kids. It gives them a good basis for choosing good food as an adult. BUT keeping certain foods completely away from them can have the opposite effect when they are given a choice to eat something not so good for them.
Teach her what foods give her energy and make her stronger. Then teach her that the foods that are not so healthy can be a pleasure to eat every now and then.