Hello, Mollypolly, and welcome to TP from me as well.
My husband was diagnosed with vascular dementia in February 2009 and he too can be aggressive, quarrelsome, just plain nasty and has been known to swipe out at me and also put up his fists and invite me to "Come and get it".
It is, of course, very distressing for your mum (and for me, but I am perhaps now more used to it than your mum), and the only good thing I can say about it is that within minutes Brian has forgotten all that was said. If I am upset by it and remain a bit tearful, he often asks for the reason. When I tell him he apologises profusely and cannot understand why/believe he could have behaved so badly. There are quite a few of us who live with this.
I would suggest your mum asks the GP to arrange contact with the CPN (community psychiatric nurse) who will give her lots of support. If your mum feels she needs a break now and then, the Social Worker will arrange for your dad to go into a local care home for a week. And if she chooses to, this can become "rolling respite" in that she could have a week "off" every two months. (That is how it works down here in Dorset, anyway.) Do not let her run herself into the ground in her caring role. It is very difficult to ask for help/respite/support the first time round.
Love, Nan XXX