It's an auto-immune disease. Her body has formed antibodies against a gluten-transaminase. Thus, when the protein is high concentration (as it is after she eats gluten), she'll have symptoms. They can do a serological test for this, but a negative test does not mean she does not have the disease. If she stops eating glutin and the symptoms subside, then that's probably the disease. This is cheaper and easier than a duodenal biopsy (which is diagnostic). It has a genetic component--but environment also plays a role in activating" the disease. Trauma and stress are known to contribute to the onset of the disease.
It's likely she hadn't encountered the conditions that triggered it.
Yes, see above.
*I am only a first year medical student. Don't take my comments as gospel. I saw a patient two weeks ago in a clinic who had the disease, and I did some research and talked with my attending physician about it.