I had problems with this some years ago.
She has been to her GP several times, who just prescribes military grade pain killers
If this is seriously debilitating then the next step would be to properly diagnose the problem. In my case I had an MRI scan which clearly showed the prolapsed intervertebral disc that was at fault. I had surgery a few weeks later which resolved the problem immediately. That's probably 6 or 7 years ago now.
However, my understanding is that the culprit is not often as clearly identifiable as it was in my case, and overall surgery of this type (microdiscectomy) does not have a wonderful success rate. The surgeon clearly warned me that there was a significant chance that it would not work - I can't recall the figures now.
I also understand that the risk of a repeat prolapse (of an adjacent disc) is fairly high.
At the time I was virtually immobilised at times so I felt I had no option but to go ahead and thankfully it worked really well.
I've followed this up with specific exercises to strengthen the muscles of the lower back at a gym that deals with this sort of thing.
My feeling now is that perhaps the need for surgery might have been avoided with proper physio, even after the disc prolapse, but at the time I needed something done fairly urgently.
My advice, then: ask GP to refer for an MRI scan to see if the cause can be visualised and take it from there rather than chucking painkillers at it. In my case I cannot recall any painkiller that worked for me aside from diamorphine.