It's not overreacting. Several schools in my area(like the SELF school), have a nut free campus. It even says on the doors Nut free zone.
I encountered a girl at my son's old school(SELF) who was so allergic, that a person in the same room with her having a PB&J caused her to have an anaphylactic reaction. The new child's parent's were informed.
Inform her that you worry, and though you enjoy her desserts, you cannot have any of them with nuts here. It may have been an oversight on her part(you know how holiday baking goes).
I modify menus for everyone, all the time, and when I hang out with new friends for say lunch, I ask first off, "Any food allergies?"
I've encountered:
Gluten
Nut/legume
Corn
Pork
Shellfish
Dairy
I also tailor a meal plan for a day if a friend is Vegan. Korean food is very flexible for that one(and I'm glad I got so many recipes for everything from my Mother, and the knowledge that they are easy to adapt for Vegans).
I have so many friends with Corn allergies, I have eliminated most of it from my cabinets. I do keep corn meal for myself(I'm fond of Johnnycakes and my morning cereal with cranberries), but it's clearly listed, and I keep it out of the way. I've also modified my recipes to not use corn syrup(harder to work with, but possible).
Your husband needs to talk with her, as do you, in a non-Holiday setting about this. There is a misunderstanding somewhere, I'm sure.
Take care, and try to relax.