The rule here is if you are here when we eat, you can eat too. Friends, students, daycare, whatever - if my kids are eating, I think it is incredibly rude to tell another child nope - you can't have any.
That said, mac and cheese and pbjs are not nutritious foods. I've actually noticed that I am hungrier AFTER I eat them than before - esp on white bread.
I would talk to the little girl's parents and suggest that she be screened for diabetes. Many Type I diabetics, before they are diagnosed, show only increased thirst and hunger after simple sugars (fruits, white bread, jelly).
And I would keep your opinion about her weight problem to yourself, unless you're ready for someone to point out your children's lack of manners or delay in potty training or SOMETHING that you feel is none of their business. At the same time, if her parents bring her to get tested for diabetes the doctor might do a full workup to check for biological issues if, in his medical opinion, her weight (as opposed to her growth) is an issue.
JMO