Try having her cut out all dairy, especially cheese. Switch to all whole grains (whole wheat bread, pitas, bagels, crackers, pasta, etc.). And lots of fruits and veg.
Have her drink 1/2 cup of prune juice or eat 3 to 4 prunes per day. Every single day. If that doesn't help, have her check with her pediatrician or the enterologist about using Miralax daily for a while.
And get the child into counseling - the bad psychological stuff she is dealing with is having a direct physical result on her body. If at all possible, get the dad into the counselor's office too. But at bare minimum, get this girl in to a counselor so that she can get some help with dealing with the trauma and the dad's awful behavior.