--that bypass operation should be investigated.

There have been anecdotal reports of neuropathy due to nutrient deficiency/malabsoprtion following gastric bypass. B12 deficiency has often been a culprit. as the body needs intrinsic factor in the gastric tract (and available surface area) to break this out of food and be absorbed properly.

See:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1483244/

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3237207/

http://www.neurology.org/content/63/8/1462.abstract

A conscientious surgeon should alert a gastric bypass patient to the possibility of such malabsorption post surgery and about the likely need to supplement, probably for life.