You could say to mom that you guys were talking about healthy eating and he mentioned he eats a lot of chocolate etc when each child spoke about what they eat at home. I would say that you are confused as they asked that he eat a healthy diet and inquire about what their ideas are for healthy eating. You can then give a copy of your menu and explain what the kids are eating and that he has been eating super healthy without issue.

If he gives his parents a hard time at home re: food (or perhaps they have just developed some bad habits) offer to have a healthy eating week as one of your themes which incorporates them tracking what they eat at home or perhaps trying a new healthy recipe at home. I've had these conversations with families that have very small children and the child is a very picky eater. One family consulted with a nutritionist and is incorporating some of our meal ideas at home.