My brother thinks my kids' diet is oddly healthy. And he loves to spoil them.
But even at age 3, my kids would request what they like. So if you asked about fries or apples, they'd take the fries, but if you asked nuggets or turkey sandwich they'd go for the sandwich, and heaven help you if you tried to give them juice instead of milk or water to drink.
And yeah, my brother likes to spoil the kids, but he doesn't want to deal with aftermath. So I would just say "Hey, can you try to keep Junior to non-fried foods whenever you can, please. If he gets fried food more than once a day, he gets digestive issues. I would hate for you to have to deal with that at an amusement park! I know there will be more sweet treats than usual at Disney, and that's fine. But you may want to give him smaller helpings and make sure he gets as much fiber as it's convenient to feed him. Again, extra sugar has caused digestive stuff before. His tummy just isn't used to it. He isn't picky at all, so I am sure he will happily eat the fruit and veggies and grilled options, when they are available."
Then my brother would roll his eyes at me, confirm with Junior that he really likes grilled chicken and fish, and give the child two times the amount of sugar I'd be comfortable with...which is about half of what he would have given if I hadn't mentioned it.