This is a difficult stage in the Dementia for your Mum she is aware she is not functioning well, but not aware it appears why. Does your Mum know she has dementia? She will get to the point where she will not remember how she used to be nor remember she has dementia but how to help and support her in between.
It will be important to communicate with your Dad so as you need to sing from the same sheet.
What Mum will need more than anything is reassurance because usually it’s fear of “what is happening to me?” That needs an answer if a person is okay being reminded “there’s a condition that is causing your memory to falter and may cause you to have blips at times” that may be helpful to her, people with dementia need to have a reason why this is happening to them , however the term ‘dementia’ is usually responded to in shock and complete denial (once a person is well past the time they were told they have dementia) so it needs to be worded in a way that she can accept without her getting upset, angry or denial, a line that both you and Dad use.
To add to your task your Mum will likely want the cause of the problem to be anything or anyone.
But feeling safe, reassured, and that someone else is there (albeit at a distance) will really help so hearing your Dad say “It’s okay I’m always here” may help also if you ring her more often, however she may become very dependant then on your calls. so a healthy balance is important.
You may find Dad too will need support, the Alzheimer’s Society have a number of fact sheets that may really help on different subjects, look on their website “Fact sheets” .