While I am sorry for your frustration over your snack choices, I can't be too sympathetic to your anger.
I'm not sure how often you have to bring snack, but it can't compare to the anguish parents like myself go through on a daily basis to find three meals a day plus snacks plus desserts that are safe for our children.
This mother, not the school, is the most informed person about what is safe for her child. Who else but her should be deciding what this child eats?
While it's easy to say that she should be bringing snack for her child (I do this for my DD) it is also unsafe for other children to be eating foods that this child is allergic to. The proteins from any dairy or peanut containing foods can be inadvertantly wiped onto toys, books, etc. in the classroom that this child can then touch and react to. That is a very dangerous situation. (We try and remedy this problem in my DD's class by wiping every child's hands after snack, but my child has a minor allergy. This may not work for in this situation.)
What this mom is doing, while occasionally inconvenient to you, is keeping her child from DYING! Isn't that worth a little extra effort of families in the three classes? Wouldn't you want everyone to do all they could for your child if the roles were reversed?
Children, especially those at the age I'm guessing these children to be, are creatures of habit and often ENJOY eating the same foods daily. I wouldn't worry about that.
I do encourage you to speak to the mom for any suggestions. I KNOW she has thought a lot about it, just as you have.
Fresh fruit sounds like a great idea. It's healthy for all (except my DD who's allergic to melons) and there are a variety of fruits from which to choose.