I'd probably just assume that it was due to allergies/intolerances in the food served (which is fair enough, it's quite common these days) and shrug and know there's nothing I can do - have party at Macca's, Macca's food gets served. I try to make people aware when inviting them over for parties/get-togethers that I'm happy to accommodate for intolerances or allergies, I just need to know what they are first so I can make sure I'm prepared something they can eat! This goes for grown-up parties as well as inviting the girls' friends over for playdates