Food sucks. Yeah, I said it. Why? Because when you are not allowed to eat a certain type of food for any given reason that is out of your control, food sucks. I'm not talking about these fancy diets where you cut out gluten or eat ice instead of real food -- this is something way more serious.
Food intolerance or allergies are a daily struggle for people who deal with them. The severity can go from slight discomfort all the way to death. There are also some who do not have to even eat the food to have a reaction. Some could just be in the same room as it, have cross contamination in some area, or even be breathed on hours later and it can cause an episode.
It's very difficult to guarantee that the place you are eating is safe enough for your level of reaction. Being a person in the situation of having to be so careful with their surroundings almost feels like a burden being put on friends and family. From restricting restaurants to dine at to what food can and can't be in the house takes a toll on everyone.
The best way to handle this is to be considerate and put yourself in that person's shoes. Realize it is not their fault. Be considerate of it and do not always put it on the person to pick the place to eat. Also realize that if the person does say no to the restaurant, do not take it personally.
If you have questions, ask. Do not assume that the person is on a diet or choosing to do so. If you are curious about why this person is intolerant to something, they rather have you ask about why they can't rather than making something up. Also knowing about your friend or family member's condition means that if something does end up happening, a reaction or illness, happens, you are able to take action in the correct way.
When I was little, I rarely had food problems. I had a sensitive system but things did not seem to bother me. As I grew up, more and more sensitives appeared. Summer of 2012, I got to the point where I quit eating because all food made me sick. That ended up being my gallbladder not functionally correctly. Once that was removed, my list of "no-no" foods doubled. From dairy, to some gluten, to lettuce -- my diet shrunk tremendously.
One thing that helped me though this is having friends and family members who were understanding of this problem. They know what I shouldn't eat and help me not to eat them (even though I do from time to time and regret it). When I do eat those foods, they know to comfort me though my sickness and once it passes, everything can resume as normal.
If anything is learned from this, let it be known that everyone deals with something, and sometimes it happens to be one of the things your body is suppose to be good at. Be appreciative of your ability to feast on whatever you'd like -- not everyone has that luxury.