Psalm 139:16 contains a remarkable claim about God's knowledge of a person that precedes and outlasts their own self-knowledge: "Thine eyes did see my substance, yet being unperfect; and in thy book all my members were written." The Hebrew golem — "unperfect substance" — is raw, unformed material. God's knowledge of a person begins before formation and is written in a book that does not fade with the person's memory. The person your parent is being reduced to by dementia is not the total record God holds. He holds the whole.
Romans 8:38–39 lists the conditions that cannot separate a person from God's love — and notably includes "nor things present, nor things to come." Dementia is a condition of the present that is progressing into the future. Neither the present loss nor the further losses coming can reach past the love Paul describes. Your parent in the late stages of dementia is still held by what cannot be separated.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.