Now I probably am being unreasonable, but I have noticed on a few occasions my friend will not order food and then will eat our leftovers.
Such as I ordered a dessert, too big and I couldn't eat it all. She had refused to order a dessert and then when I finished she took a spoon and started eating it herself.
Another time me and a friend had a sharing platter and again didn't finish it and she started helping herself to it.
I guess the food is just going to waste so she might as well eat it but I do think it's quite cheeky still.