He knows he's wrong and he's flailing wildly at everything he can to justify lying to his friends.
He admitted it was originally a mistake, which means he wouldn't have put it in if he thought about it.
Because he wouldn't have, his justifications about chicken stock being some absolute bastion of home cooking that transcends basic human decency are moot. Feed people what you say you're going to feed them, or don't abuse their trust. Simple as.
His argument boils down to: "Oops. I accidentally made this non-vegetarian. I don't want to start over, but I don't want to feel guilty either. Chicken stock is a super important part of every meal that I make! And it's not like it's /real/ meat. I mean, yeah, sure, it has animal components in it which goes against the very basic tenets of vegetarianism or veganism, but it's not like it's got a /lot/ or anything. And I'm really actually doing you a favor because you're a vegetarian and vegetarian food sucks, and instead of taking it as a challenge because I clearly have a sky-high opinion of my own cooking, I'll resent you because I can't eat half a cow at one meal.You won't even know the difference if I don't tell you (even though, it's like, a super special amazeballs ingredient that everything is lacking without), and you'll probably even /like/ it. Besides, I bet John and Marsha will be here, and they eat meat. So really, you're stupid for being vegetarian and I'm awesome for feeding you animal products. Done!"
I have no quarter for people who decide that instead of being honest or truthful ( "I don't like cooking vegetarian fare. Let's either go out, or do something that doesn't involve food."), hide things and snicker to themselves as they intentionally go against their unsuspecting friends' wishes.