hi there and welcome
Does your mum have any help at all with your dad?
She can ask for a carers assessment from social services (this will give her a few hours to herself when they put in a 'sitter' to be with your dad) and an assessment for your Dad for things like a day centre, carers or whatever they/you/your mum thinks will help the situation. Phone the duty desk at your council adult care services and ask for an assessment.
Is she getting attendance allowance for him - needs to apply for the higher rate is she is caring day and night? It is non means tested.
Good idea to phone the local carers organisation and ask for help - they will probs send someone round who will be much more helpful that social services and help her with all sorts of local information and also with form filling - google your area with carers organisation. There is probably a lot going on that you don't know about yet and there is a lot of help out there.
Lastly but not least PLEASE have a look at this
http://www.alz.org/greaterdallas/documents/CompassionateComm.pdf
it is no good reasoning with your Dad - it won't help and it will make everyone frustrated and he is probably talking about his parents because he is afraid, losing your memory is a VERY scary place to be and that sometimes triggers people to talk about their childhood and a safe place to be.
Good luck, lots of help and support on here never be afraid to ask ANYTHING! take care and keep posting xx