@HarukaNazu I read it as the initial request was the most impolite part. He shouldn't have asked to eat her sandwich in the first place...he could've asked her if it was good and ordered his own then. Considering he did order his own (and extra) it's odd to see him go through all the trouble of eating her sandwich first, then comping her, meaning that there's half a sandwich sitting wasted on the table (maybe it's just because I hate food waste, but she just said it's too much so I doubt she'll eat it..? and she already took a bite so he won't eat it...and seems like a situation where she won't get a to-go box).
The latter parts where she's irked is a little petty, but it's true him asking to eat a sandwich she paid for in the first place (without saying up-front that he would comp her) is rude.