I get severe mind numbing pain under my right rib caused by my gallbladder. It is usually follows nausea and bloating for me.
Does she seem to have these pains more often after eating anything, specifically something like fried foods, eggs, or lots of meat/dairy. Has the doctor's ruled out a gallbladder problem? Also a clear sign of a gallbladder problem- yellowish (floating) stools.
Does she have any problem with diarrhea or constipation, perhaps it could be a food intolerance.
If she gets a high fever, looses appetite, and/or starts vomiting everything you should take her to the hospital as those could be signs of an infection or even appendicitis.
I have a bicornuate uterus also, and it never caused me pain under my ribs (or anywhere else). She might have endometriosis, or something else. Keep looking for an answer, it's not the shape of her uterus.