Is she actually dying or is this just a generalized fear of death? When my mom was feeling this way I told her that I would make sure she was cared for and not in pain and that she wouldn't be alone. She may find comfort from speaking with her spiritual counsellor, or a hospice nurse who can answer any questions she is mulling over. Medication for general anxiety might be helpful as well.
Can you say that you accept what she is telling you is the truth - and that she can close her eyes, and trust that you are right there with her watching over her...protecting her?
but without more information its hard to say what you should do.