When you are over the shock, it will be worth sitting down with your mother and siblings and looking at options, first to ensure that your father is able make use of and stretch the capacity he has, but more importantly to help prevent too heavy a burden falling on your mother.
All sorts of options, including a move sheltered housing where your mother would not be the only spouse who is a main carer; a taxi account if your dad had previously done most of the driving: Just Go coach holidays or similar are very tolerant of people with memory problems (and an easy way of having your parents picked up from their home and stay in a hotel somewhere near you!), and and and. If your mother has now previously handled the finances, it might be an idea to suggest a POA or a third party name on the account, which would allow you, with internet banking, to look after this side of things.
The early stages of dementia are very difficult, as your father will be frightened, and trying to deny or fight it. I found my mother's never ending and repeated questions really trying, but understand that she was wanting to make sense of the world around her. She has moved onto a level of acceptance, which is probably why she is less stressed and happier.
As well as the emotional side there are lots of practical problems, but plenty of us working our way through them.