I'm truly sorry to hear that Penny. Dementia is a terrible affliction, and you, your mom and his entire family must all have been devastated. Alas, the last time I saw my dad happened to be only a few hours before he suffered a sudden, fatal heart attack. He was also in his mid-seventies at the time of HIS death ... which was totally unexpected, in the sense that he'd always been a very fit and active person.
More than two decades have passed since, but he's still very much in my thoughts ... especially on Fathers Day!
God Bless.