He only eats McDonald's fries

I recently took up a babysitting job for a young boy. His mother works overnight 2 nights a week so he needs dinner while I'm watching him.

He literally eats McDonald's french fries and a large coke every single night. He doesn't eat breakfast or lunch because of his medication; lack of appetite. This seems like laziness on the mother's part. She told me not to bother introducing new foods because he will not eat them. "He's been on this diet since he was 4 (he's 10).

Their pantry is full of chips, cupcakes and candy. Refrigerator is full of soda. She's a nurse too, shouldn't she be teaching healthy habits to her son?

He ironically is not fat. He's actually bone skinny and pale, very pale (washed out and almost ill looking). He will cry if I suggest to eat something other than crap food his mom buys him. Guess I just have to deal with it but this seems like neglect on her part.