The effect of back pain on movement and function seems to be linked more closely with duration of pain than intensity. Pain doesn't have to be severe to affect a person's quality of life if it lasts long enough. In other words pain intensity doesn't predict disability as much as duration.
A recent study from Spain surveyed back pain patients on their first visit. They repeated the survey 14 and 59 days after the first visit.
Quality of life declined over time for low back pain patients. Half the patients were rated as "disabled" by their back pain on day one. The percent of disabled patients increased to 65 percent by day 15 and 73 percent by day 60.
Treatment to focus on improving function rather than decreasing pain is advised. A referral to a pain clinic might be a good idea. Now might be a good time for re-evaluation by her doctor.