Well, in THIS respect, I think it should be a broad range issue. That there are certain behaviors you expect her to follow, whether at home or away from home. Protecting her health is one of those behaviors.
However; everyone makes mistakes. It happens. And no matter what, she should be able to come to you for help, or advice, or just to unload :P You'll love her, and support her needs.

In our house, honesty is the number one rule. And we have a "If anyone ever says 'Don't tell' what's the first thing you should do as soon as you can? Tell Mommy or Daddy about it." discussion too. It applies to anything.

My dd has told me about bad choices she made, and I try to always take her seriously and nod and listen through (even when we're discussing the fact that she feels guilty for using Dale's pencil when Dale was sick, to write her name. The way she presented it I thought she'd tried to burn the school down or something.) It's the same with food. She gets sick if she eats the wrong thing...we discuss it and if she made the conscious decision to eat a blue candy due to peer pressure, she doesn't get the same sympathy she does if Grandma told her clam chowder was dairy free.

I inform the adults she will be with of her dietary restrictions, and then expect her to monitor herself. But, her sensitivities don't cause physical damage, to my knowledge. She's 8 BTW. My 4 y/o is allergic to nuts...adults get the "Nuts are potentially fatal" talk for her. I'll probably take more of a role in protecting her, because anaphylaxis scares me!

Anyways, there are choices everywhere. Some are big, some little. Some are HUGE to kids and hardly a blip on our adult radars. Food is just one of many things we will be trusting our kids to make good choices about...and its probably one of the first ways they will learn to take charge of and protect their bodies.