--the usual figure given is that up to 20% of individuals with biopsy-proven Celiac dysfunction of the intestinal villi have negative Celiac panel bloodwork. So it's possible that she does have that issue, or at least gluten sensitivity, if not full-blown celiac.

I see you've already found our Gluten/Celiac forum--specifically, I would read in The Gluten File (which is in the Useful Websites section at the top) the section on diagnostics--lots of info there.