OP I can understand your frustration. My DS has an egg allergy, as well as dairy intollerance. I picked him up from daycare once and the teacher came to me and said that all the other kids had sponge cake, and she felt bad for DS so just gave him a really small slice... I was so angry. The poor little man ended up with a belly ache and a rash around his mouth all afternoon. I took him out of that daycare and have put him in a new one now. I constantly remind them about his issues, we have an action plan for his allergies and his asthma though which were required before he started and they are really good at the new centre.
I would speak to the teacher and make sure to stress the importance of her diet. And I would inform the teacher that if he/she cannot adhere to this you will speak to the principle etc to get some compliance.