I think parents who do that do it because of their own personal tastes. Some people hate milk so much that they think they are being cruel to give it to the child. They’re wrong. But what can you do?
As for the rest of your daughter-in-law’s diet program, I have to wonder if she herself is overweight. Is she?
No, she eats good food, cooks good meals. Just won’t make her son eat because he doesn’t like good food and throws a a dirt. He will eat Mac and cheese once in awhile and that’s it.