Vitamin B is involved with the cellular use of insulin and the transport of glucose across a cell membrane and into a cell. Diabetics that are quite insulin resistant or do not have enough insulins in their body may have abnormal Vitamin B levels. I cannot answer specifically about B12 as I do not know. I would expect that most doctors would treat the insulin resistance or high BGs and expect the persons vitamin B levels to normalize.
No doctor I have ever spoke to has recomended any vitamin supplements to me because I was diabetic. That said, Vitamin B is water soluble so you cannot overdose on it (unless you go super crazy with it) so if you want try I doubt there is much harm.