Good strategy for today!
Ok this is exactly why you need a 504 Plan. Not child's fault -- well -- what ARE the regulations in this class for shared food?
Provide the articles about the recent DEATHS and child saved thru extreme measures (days on a heart/lung bypass machine as a miracle to save his life). Out-of-routine shared food events are very risky for young children with LTFA. Exclusion is not OK. You want SAFE SHARED TREATS where you can check labels of all food shared in the class (yes, labels, so NO home baked treats with hidden ingredients and cross contamination in home kitchens!) or NON FOOD CELEBRATIONS (this is easier for you if they'll do it!).
We can help. Start with the 504 Request. Document not only the event, but also that the principal did not handle it appropriately (ie, however you can say that effectively, but do document it, so the principal knows she has to work WITH you on this or she will be documented every time!).