I can see why anyone would be frustrated if they'd spend time and effort creating a different-than-normal menu for a group of people with a specialised diet. I can see that they'd be a bit pissed off if those people then requested something outside of that specialised diet, because then clearly either their principles or their reactions aren't that strong.
I can even see why someone would post on social media, venting their frustrations and exposing the illogical/irrational/fussy/hypocritical/attention-seeking or whatever you want to call it behaviour.
This would all be ok if it was just the mozzarella on pizza.
If, however, she DID actually spike a vegan's food with meat, then that's really not on. Same as the chef who put bacon in a PP's beans. That's dishonest, disrespectful and very stupid.
As an aside, it IS possible to have an allergy to meat - apparently it can be brought on by a certain type of tick bite as well. So not all vegetarian/vegans are this way out of choice (although most would be).
But the hyper social media reaction is just ridiculous. Gerald Ratner moment for her, probably - but she in no way deserves threats of physical violence or intimidaton.