“My mother is very fearful of death and she needs help?”

Therapy maybe helpful

There could be some underlying issues causing this - such as medical issues. In the elderly they may have urinary/bladder or kidney infections that can cause anxiety or emotional changes. Perhaps it’s others in her circle of friends or family that have passed away and her fears are very real. She could benefit from therapy - meeting with someone to address her fears. If the fears are persistent and impact daily life - she could benefit from meeting with a mental health provider and determine the best care for her.

I would recommend a book by Thich Nhat Hanh called "No death, no fear."

If she hasn't responded to your efforts so far, I would recommend finding a good therapist who works with the elderly to further evaluate and deal with this issue.