This is not a particularly festive topic, but a friend of mine is on the verge of losing her mother, and I wanted to pass on the same advice I have to her, in case anyone else is in a similar position.
First of all, I must highly recommend this site: Fish Eaters – The Catholic Way of Dying. It contains very useful prayers, including the Commendation of the Soul to God.
Secondly, loved ones should make use of their fear and grief by offering it up as a prayer. This is hard; the natural response to suffering is to try to reduce it by any means possible. However, as Catholics know, suffering has a redemptive nature. You could, for example, offer your own pain for the good of the soul of your loved one. This could mean less time in purgatory, if any, for those whose spiritual lives are healthy. For those less secure in their faith, it could be the difference between Heaven and Hell. Furthermore, accepting suffering rather than fighting it means you can invest your energy elsewhere, like praying, or comforting the dying person.
Thirdly, remember that what is eternal is what matters. All we experience here is fleeting, and a means to the end of seeing God face to face. It does not feel like it at the time, but this is the truth. Therefore, take heart, and rest in God, knowing He lives in us and we in Him, so long as we try to love Him.
Fourthly, try to keep in touch with a good priest. Priests can advise on ethics, prayers, and even just practical matters like funeral arrangements. Having a person slightly removed from the situation can help keep you grounded when you feel grief might swallow you.
I realise this advice is all fairly black and white. Death is a miserable thing to watch. Waiting for it is terrifying. This advice will, however, help to make the death as good and holy as it can be, and knowing you have done all you could to help the dying, even if it meant more suffering in the short term, will bring long term consolation.
God bless you and keep you.