Brief Answer:
more studies needed
Detailed Answer:
in a young female my first thought would be to investigate gynecological sources of her anemia. If her menses are regular and not heavier than usual, I would still suggest her getting an ultrasound of the pelvis and abdomen.
Gastrointestinal causes of anemia are also obvious good considerations, and i see she had a negative endoscopy and
colonoscopy. The next step would be a Wireless
capsule endoscopy (if available at your gastroenterologist office), as this would examine the small bowel which is not easily accessible via endoscopy and colonoscopy.
Celiac disease needs to be ruled out, however with her severity of anemia, i would expect her to have diarrhea (from
malabsorption). Crohn's disease (or
inflammatory bowel disease) should also be considered, and again a wireless capsule endoscopy would help.
If all of the above is unremarkable, i would suggest you taking her to a
hematologist (blood doctor), as she may need a "
Bone marrow biopsy" to see if there is a problem with her body not producing enough red cells, or if her body is destroying her red cells. Many disease can cause this problem. I see that you stated that Leukemia has been ruled out, but if a bone marrow biopsy has not been done she may need it.
The fact that she has FOBT positive for blood makes me think it is something gastrointestinal, however keep in mind that this test is not a 100% accurate, and sometimes test becomes positive from blood coming from hemorrhoids, which usually can't explain the anemia.
Hope that helps, let me know if you have any further questions or concerns.