That is indeed a sad thing, I hope that she is able to get some relief. A doctor to evaluate why she is having the pain is important. Treating the pain matters but what is underneath it. Doctors are very careful about addiction and that is actually in your girlfriend's best interest as developing an addiction to pain pills is a whole new issue. The idea would to be short term treated while discovering the underlying cause and 'fixing' that. I fell down stairs and broke my tail bone once. It's very painful. They don't do anything for it other than allow it to heal which takes some time. Sitting is painful. Does a back brace help? Anyway, I'd see an orthopedic surgeon and/or neurologist for low back pain and physical therapists can do amazing work.
I am so sorry. No one deserves that. See if she can see another doctor..any new perspective would be helpful.