Do you have a MRI report or know the details about her back/disks, what levels of the spine, how many are affected..
The report should have the specifics. The terminology in the report can be helpful..it may also indicate lesions & MS related information..
If lesions are in the same area of the disk issues, maybe that is the specific reasoning..

Our Spinal forum might be the place to post/read about about specific spine issues, sometimes surgery does not stop the pain unless nerves are being directly affected..is there severe stenosis or DDD also?
How severe is her MS if the spine/disks were not involved?