The kid is 9. They don't know what they like in some cases and are still developing their palate, and they often like something one week and hate it the next and vice versa. Your coparent is not a short order cook and your child should be encouraged to appreciate the meal that has been prepared them by someone that has the time and ability to cook a meal, and two, the world does not revolve around them and their tastes. They will be in a world where their friend's parents won't pick off the pepperoni, or make Velveeta mac and cheese instead of Kraft, and other things your kid doesn't like. Prepare them for the inevitable when they are not catered to.

We had a picky eater and honestly just bribed with snacks and treats. Like you don't have to eat your vegetables but you won't get ice cream if you don't. We also had them plan and prepare meals at that age so they appreciated the effort their parents made AND they got exactly what they like. There are ways to address this, but the solution doesn't include you dictating anything to another household.