Is it pretty unlikely that she would have had a false positive to the DGP IGG?
Very unlikely. The DGP IgG is 99-100% specific to celiac disease. It is the MOST specific of all the tests. Look at Table 3 on page 12 of the World Gastroenterology Organisation Global Guidelines (for) Celiac Disease. A positive DGP IgG is as close to a sure thing as you can get.
I'm glad that she isn't doing too badly. Good luck with it.