Proper health and nutrition are very helpful in mitigating the symptoms of any mental disorder, but B12 won't cure or medically make better any mental disorder. It can only provide nutrition a person may or may not be lacking. It's completely possible to have too much B12 in the system and not everyone tolerates it well, so I'd be careful.
I agree, always check for proper nutrition and hormone levels. However, that will ensure physical well-being. Any true disorder will likely require more therapy.