Sry for your loss,
No body can comment on the reason without knowing the complete history of the patient.. Heart disease can range from heart attacks to heart blocks. Other diseases like
diabetes or hypertension also play a important role.
Without knowing the details of her medical history and the details of the event leading to death No body can comment on the exact reason.
As for a pacemaker, it only works to increase the heart rate if it slows down.
You can read more about it here
https://medlineplus.gov/ency/article/007369.htm