If she's completely gluten-free now, a blood test for celiac disease may or may not show anything - the antibodies go down over time with a gluten-free diet, that's how we monitor progress. DO NOT take any advice that says "Go back on gluten for 6 months and then we'll biopsy you" - if she really has celiac disease, that could make her very ill and sometimes takes years to recover from.
I use an IgG food panel by U.S. Biotek in my office to find milder gluten-sensitive patients, but it's not a standard test and your GI won't know anything about it. Same problem though, if she's gluten free and it shows negative for gluten, it may just mean she's doing a good job of being compliant with her diet.
The most common food sensitivities I see with gluten issues are dairy and eggs. Also, there's a big overlap between celiac disease and thyroid disease, and chronic constipation can be a symptom of thyroid disease, so she should get that checked.
You can ask for a gene test (HLA DQ2/ DQ8) to see if she's at risk for celiac disease. It's imperfect, as 30% of the population has at least one gene and they don't all develop celiac disease (and it's possible to have some gluten issues without the gene). Still, if she tests out like me (I have the worst possible combination, 2 copies of HLA DQ2) she'd know she should really probably stay off of gluten. (Given that I have these genes and have another autoimmune disease already, MY risk of progressing to full-blown celiac disease in my lifetime is 30% !!!).
