Dear Paul,
Welcome!
Your question is such a hard one to answer, and whatever you decide, you will never know whether the decision was right or wrong. Would your Mum understand the implications if you told her the diagnosis? Would she be frightened or depressed? Or do you think she (and indeed your Dad) might benefit from the help and support of the Alzheimers' Society, carers, friends and family, who would then be able to openly discuss her problems and strategies to overcome obstacles that arise as time goes on?
My husband was diagnosed 3 1/2 years ago, and he has been on Aricept since then. We as a family decided not to tell him that it was AD, as we did not want to confirm his worst fears (that he would 'end up like his mother'). He is not a positive person, and we worried that he would get too depressed. I am not sure that we were right: his depression set in anyway, because he knows that he is not 'functioning properly', and gets very frustrated and angry. But because we did not tell him, we are unable to openly introduce anything to do with the Alzheimers' Society and their care schemes.
He still has good days, but if we told him now, he would not understand what AD means, and so we will never know how he would have reacted had we been open and honest in the first place. - It must be such a worrying thing to realise that you don't quite understand the world and the people in it any longer ....
Perhaps you could try and enlist the help of the Community Psychiatric Nurse. I did not realise until recently that she is an obvious link who should be aware of what is available in your area to support both your Mum and your Dad.
All the best, Carmen