I used to be more dumb about this stuff, but really if your friends have any dietary restrictions (not limited to being vegetarian) they don't have to be perfect and you shouldn't interrogate them about this stuff. Their dietary restrictions - for whatever reason - are not passing judgment on you (unless they say so) and they'd really rather have a meal without being asked about it. Related is getting mad at fat people for being fat and then being weird about how they try to navigate a diet without being a recluse. Meals are a social thing and demanding people diet is demanding they violate social customs.
Many vegetarians eat a piece of chicken occasionally. Let it go. Eating is a fundamental human need and whatever choices people make they don't want to be asked about them constantly.
I am not a vegetarian.
