Is your grandmother taking a cholesterol lowering medication; lipitor, crestor, zocor, etc? These are almost universally given to older people and can cause the weakness leg pain and inability to walk that you describe. They will also inhibit healing from the back surgery (and cause back pain too) and cause gastrointestinal pain and problems. Some of this is available on the particular drug product monograph available at the pharmacy. In the big blue book called Physician's Desk reference. Demand to see it if any of this sounds right.
Many physicians do not know these drugs can cause such problems, and also do not know the drugs have no proven efficacy in the elderly, or women.
Statins.
I'm not an expert by any means, but I've read that pain in the legs can be a circulatory problem of some sorts. Also, after I went off the bc pill, I developed lung blood clots (pulmonary emboli). They tested my legs to see if the clots had originated there (deep vein thrombosis), because I had severe cramps and paints prior to the discovery of them in my lungs. This issue can be caused by several things such as old age, obesity, factors in the blood or being prone to it, birth control medicine, and long periods of inactivity (i.e. long airplane trips, bed rest, etc).
I don't know if this would help you any, but that's all that went off in my head when reading this.