My mother is afraid of dying.

I'd venture to guess that 95% of human beings are afraid to die. What does coughing at night have to do with fear of dying, I don't get it? Get her to the doctor to see what medical condition is causing her coughing at night, and then pick up a few books about near death experiences to calm moms fears a bit about dying. Proof of Life by Dr Eben Alexander is a very good one, available on Amazon.

In the meanwhile, prop her head up with extra pillows and see if that helps.

Good luck.

Maybe try elevating the head of the bed slightly, you can put a bolster under the mattress or blocks under the legs of the bed.

First you make the adamant statement that she IS afraid of dying.

Then you say she coughs a lot at night.;

What, pray tell, has coughing a lot at night have to do with fear of dying, and why would this come into your head?
I would consider that it is perhaps YOU who are afraid, if she has not expressed such a fear.

Now onto the coughing a lot.
Can you ascertain any reasons for this yourself?
Have you any ideas?
Sometimes when a senior has trouble laying flat without a cough or without some shortness of breath it indicates that there may be some Congestive Heart Failure going on. Or there may be some underlying lung problems such as COPD.

I would have your mother see a doctor and report her symptoms.
I think fear of dying has little to nothing to do with this unless you are not telling us something crucial to this question.

I wish you and mom the very best.