A few thoughts. It is never easy to give advise via this type of format - much better to have client in your presence. That being said, consideration should be given to a few things such as a neuropathy involving the cluenal nerves or iliohypogastric. Look into ADL's for possible inciting factors. Is belt worn too tightly, or a tool belt, work unigform, etc? Is there any behavioral cause, some repetitive motion, posture or man-machine interface that can apply subtle pressure? How old is their car and what is the quality of the upholstery. I used to keep my cars for many years and the wear pattern inthe upholstery was an inciting factor. Do a web search on Dr Maigne, the French physician who practices manual medicine. He is brilliant. He has written on a variety of topics to include T12-L1 junction syndrome. Screen that articulation and the 12th rib. Motion dysfunction in that articulation is typically an inferior glide fixation which responds to gentle superior glide maintained 2-5 minutes by just holding constant force of about 10-15lbs in a superior and slightly anterior direction (to be scientific one would use other terms such as Newtons, but being pragmatic you can easily practise on a bathroom scale - see studies such as Matayas and Bach in Physical Therapy 1992 (?)) Then check for medial and lateral glide of the rib,as well. Screen for trigger points, especially in obliques and quadratus L. If it does not quickly respond to intervention additional medical screening warranted.
Jerry Hesch