I wish the world were such that you didn't HAVE to explain or justify your existence, but alas that is not the case. I sometimes think the only dinner conversation I ever have revolves around my diet! With that said, though, here is how I try to explain.
The name of the game is chronic inflammation. Chronic inflammation leads to chronic disease. Even the tiniest amount of gluten will set off the cascade of reactions in your body that will lead to inflammation and disease. By avoiding gluten, you are not only trying to avoid feeling ill in the short term (like a stomach ache or diarrhea if you get those symptoms), but more importantly, you are trying to keep your level of chronic inflammation below the disease-producing level. Any small piece of gluten can cause an inflammatory reaction that can take up to three months (I believe that is the accepted time) to calm down–and all the while that inflammation is causing damage and making you feel less than good.
Your friend's allergy is different in that the cascade of reactions in his body from milk products releases histamine which causes the throat-swelling, etc. Histamine is rapidly cleared from the body and the immediate symptoms subside. Obviously, his threshold for histamine release is above the “trace” amount. His allergy and your gluten intolerance are two totally separate animals.
If size is the issue, have your friend think about viruses and bacteria. They are incredibly small, yet can do so much damage in the body if perceived as “foreign” by the immune system.
I hope this helps you to enjoy your vacation!