Your mother is apparently suffering from diabetic neuropathy which is quite common in patients with long term diabetes.
Lyrica (pregabalin) is meant to relieve neuropathic (nerve) pain irrespective of cause. Hence it should be used only if pain relief is desired since it has no curative role in diabetic neuropathy.
Methylcobalamin, also called mecobalamin, is a vitamin B-12 substance and is sold under the brand name of Methycobal by Wockhardt. It is indicated in the treatment of peripheral neuropathy. The dose is 1 tablet 2-3 times daily for about two months. If there is no improvement, it should be discontinued after three months.