While I don't really have a problem with anyone taking "their" sundae or other food item from F to a companion in Y, I was on a domestic transcon flight the other week were a couple was traveling in F with two children sitting in Y (they had another relative or other sibling sitting in back with the children). The parents brought their (very rowdy) kids up to F to clog the aisle by them, proceeded to give them two sundaes, then asked for two more sundaes from the flight attendant, as the kids returned to Y. The flight attendant obliged, but later another, seemingly more experienced flight attendant came out and told both the parents and the first flight attendant that they were not allowed to take F food back to Y.

They were in the middle of the cabin, as was I, so I'm not sure if another F passenger was denied a sundae later because they ran out, but it's quite plausible, as most of the passengers I saw had sundaes. It seemed wrong on multiple levels-- It's one thing to give your food to your kids, but demanding 4 sundaes for 2 F seats seems a little ridiculous; depriving another F passenger of their sundae (or other preferred food item) is even more so.