Oddly enough it was in the press today, that half a glass of wine a day, can help stave off this disease, or at least slow it down if you already have it.

Without wishing to sound awful, it may also be possible that your Mum is the reason he wants to drink in the first place?

Would it be possible for someone to sit down with her and explain the nature of this terrible disease, that nothing she does can stop what's happening, and that a little drink may be of some comfort to him.
If this doesn't work, then she may benefit from some medication for herself, as I would imagine that she was scared of what is happening, and is frightened of further deterioration - which is understandable. No one likes the unknown do we?
I wish you a happy conclusion to your problem, but involve your doctor, who will probably be able to address one or even both aspects of the problem.