coeliacs

I have to plan for every possibility with this disease: extra food? Check. medication in hand luggage and in suitcase? Check. Printed list of nearest hospital in case I suddenly go from remission to a flare-up by the time I land? Check. You don't even want to see the size of my hand luggage.
We all know someone who can't eat something. Whether it's that the food 'doesn't agree with them', 'can't stomach it' or 'gives them a headache'; the majority of people know their body. What do we do when someone shares this with us? Do we tell them there is no such thing and ask if they've been tested for an allergy at the doctors?