HI Funnys - I'm sorry to hear your Dad has had this DX, alzheimers can be a frightening word because it's a bit of the leap into the unknown for relatives and sufferers.
My own dear Dad had had alzheimers for eight years before he died last year at the age of 91. It is tough to cope with, but actually getting the DX helps because then your dad will be eligible for assistance - financial probably and SS care if he needs it, and he may be able to go to a memory clinic - where they do exercises and activities to help people.
Aricept helps slow things down, and they will keep testing him to see how well he is doing on it.
There's such a lot of things to do with this illness, that it's hard to think of what to write to be helpful. My mum joined a carers group which gave her contact with other people in a similar position - so they could give emotional support to each other, and also there was lots of information about the kind of help she was entitled to (for instance some areas have a sitting service where someone will sit with the person with dementia while you go and do a bit of shopping etc. - for a very minimal cost, also help is available with transport to hospital, and there may be day centre facilities available for your Dad, so you mum can get a break)
I would say that for a good six to six and a half years of the eight he had after DX, my Dad had a pretty good quality of life - he was happy, although a bit anxious, he retained his sense of humour and his character, he was very much still Dad, even though he might not always remember who I was. Once he reached the advanced stage, there were more physical problems, - and he lost a lot of mobility - but again, I would say he was not unhappy.
Different people are affected differently - so of course I can only give my experience. We found practical things like putting the day and the date up on the fridge so he could see it, and writing messages on things to remind him where things were or whatever, helped to some degree.
Sorry this is a bit of a ramble, but there is a lot to say - Anyway -if you have any questions you think I might be able to help with, just ask 