I understand your frustration, and that it feels invasive.
I have seen the other side - a friend's daughter had severe anaphylaxis from peanuts, and he could not sit on an airplane next to anyone who ate the bag of free peanuts. The "dust" from the peanuts could get on his shirt or skin, and then when he came home and hugged his daughter, she would react. They could not go to any restaurants because some peanut products might have been used there (e.g. peanut oil for cooking, things made in places that also made peanuts). They had a sink installed in their foyer, and people had to wash when they came in. They were not being overly protective - this was recommended by their child's allergist. It took 2 years of nutritional support to get her numbers down to zero - they still don't feed her peanut products but at least she can go to school, birthday parties, out to restaurants, they can travel, etc.
Another friend had a child with 60 food allergies - the "usuals" you would expect, but also many fruits (cantaloupe, pineapple), lunch meats (there goes that argument about meat sandwiches), and so on. It was miserable until they found a dietary answer to relieve all those issues.
As much of a hassle as this is for you, imagine the terror and misery on their side of it. And as someone else said, I know I would feel horrible if something I did caused a problem.
I have a friend who is a kindergarten teacher - you would not believe what's going on in our schools today. Allergies to peanuts (legumes), tree nut (almond, walnut etc.), dairy, soy, and more. She has 5 year olds with diabetes, ulcerative colitis, bi-polar disorder and many life-threatening allergies. She's not allowed to approach the parents about any nutritional therapies because that's "crossing the line."
We need better solutions and better alternative therapies, and better publicity about the things that DO work. I'm always happy to help people, as are the parents I've mentioned - but really, how can I reach them?
Meantime, what do you suggest that the parents of that child do, short of asking for everyone's cooperation. What would YOU do if it were your child?