Even if you take out any mention of veganism, I feel like this still isn't acceptable behavior. Restaurants and cafés are assumed to have a de facto "no outside food" policy. If you go into a café, order a coffee and pull out your own lunch, you'd typically be asked to leave.

Is it extra rude to bring flesh into a vegan establishment? Absolutely.

Yeah, I feel she like she pulled this stunt to gossip to her carnist friends about "those mean vegans are unreasonable"

she should IMHO have to pay for the plate too. some vegans are not even vegan by choice but by allergy, and just fragments of scavengers "food" -really not easily to "completely" clean from plates- can cause very bad allergic reactions and i would expect a pure vegan (not necesarily a "vegetarian") restaurant to explicitly "throw away" (as in never use it again for guests) any plate contaminated. luckily i am not a vegan myself and vegetarian mostly by my own choice, but i know how hard allergic reactions to non"food"-contaminated-food can be.

of course a sign that "meat is not allowed to be eaten there and everything contacted with it has to be payd to buy anew by the offender as it could not be used any more by the reastaurant" should better be placed somewhere to on one hand help service personnel with such guests by clarifying things and on the other hand to give more security to those guests who are not vegan by choice.