Going to try and keep this as short as possible.
Person A invited a group of people over for dinner, including person B who is a vegetarian. A didn't know B was a vegetarian at the time, but B let A know when accepting the invite. A said this was fine. A made lasagna for everyone for dinner, subbing the meat out for roasted veggies for B to make a separate dish. When dinner came round A explained to B what she had made for her and explained what it contained including parmesan cheese. B said sorry, she could not eat it as parmesan is not vegetarian. A said B should have really told her she could not eat parmesan as A thought vegetarians could eat cheese and wouldn't be expected to know these things as she hasn't cooked for vegetarians before. B said lots of things are not vegetarian that aren't just meat/fish and it would take forever to make a list of all things including which cheeses she could/couldn't eat. A thinks B has been very rude and B thinks A has been a poor host.
So who was being unreasonable? I am aware this is pretty much a non-event and should not have escalated in to a big disagreement, but I am interested on general opinions.