I don't watch Casualty any more but I remember Duffy from her first stint. I suspect unfortunately the dementia will be downplayed, and she will conveniently die before anything too unpleasant happens. I just don't think prime time soap audiences want to watch distressing scenes of later stage dementia.
The best depiction I've seen of dementia was Ashley in Emmerdale. The episode from his 'point of view' was superb. He did however conveniently die fairly quickly, and while I know each person is different it is unusual to go from diagnosis to death in 18 months, so it gives a false impression of something quickly over.