Denial of a spouse's dementia is both normal and common. Sometimes it also can be a very positive defense to averting depression or even despair for the spouse. You need
to ask yourself, "What would it be like for her to admit he has dementia?" and identify the answer before you open a conversation. It is never effective to confront denial as it just increases ones defenses. It takes time for a spouse to process a dementia diagnosis and although there is variation among spouses in how long the acceptance process is, after a while reality begins to chip away at the denial.
Of course, if your father has significant care needs, they cannot be neglected. If his dementia is at a moderate stage and he requires assistance and supervision, your goal would be to see his needs are addressed"¦..despite her denial. If he is disoriented and is allowed to venture out on his own, your goal would be his safety. Thus, you could register him with a Safe Return Program (Alzheimer's Association) and present him with a bracelet as a present. If he has hygiene needs you could help him dress or wash and model "˜caregiver' behavior for your mother. If he is still being challenged to manage the family finances and cannot do so adequately, you can join with your mother on this issue.
Sometimes a generic conversation about Long Term Care decisions (like Advance Directives) is a non-threatening way to open to a conversation about care needs. If your mother allows no entry into her reality, perhaps having this conversation in the presence of their physician or a third party (relative or psychologist) might facilitate progress.