B12 deficiency --by itself -- causes neurodegeneration and contributes to dementia symptoms. Add that to the HD gene -- and it is likely to increase HD dementia. There was a recent Alzheimer's Phase 2 study suggesting that Vitamin B supplements (which included B12) decreased the rate of progression.
Getting B12 by shot or pill is important depending on the cause of the deficiency: if the person can't absorb B12, then giving a pill to supplement won't help. That person needs the shot. In my office this is a nurse visit -- so in and out quickly. Depending on your mother's ability to get out -- this is something a visiting nurse can do too. Hope this helps, L