Dear bleumonster
I would insist that you get her tested for her gallbladder. You have to take control of this situation as your doctor suggests. You are the parent not her. Take the reins and get her diet under control.
You could have her tested for gluten and dairy. This would help with getting to any food issues. Enterolab.com is a consumer lab and you can order the stool test.
But I would definitely have her gallbladder checked but her doctor is right in getting you to get in charge of her weight!!!!
Hope this helps
Brenda