This is very difficult. However, perhaps they have had a good life and so we can remind them about what a good life they have had and to feel thankful for that good life. Most people like to talk about themselves and sometimes reminding them of the good things they have done can still help someone who doesn't have a belief in a next life, because it helps them perhaps to focus on good things. With people who have no belief system, it is very difficult when facing death, because all that they think they are is going to end and this will often bring fear to a lot of people. Unless they are hoping for the end of it and they are hoping for the annihilation of themselves. Since I have not actually been in the position of having to comfort people of this type, I am not very skilled in knowing about this. So I would probably try a few things that may help someone I know who has a belief system, thinking about the good times in their life and trying to live each moment until such time as there is the end of it. I certainly wouldn't like to be in the position of comforting a person who hasn't done any good in their life, dealing with the fear would be very difficult. But I would probably try some Dhamma techniques to see if they would work. I know everybody likes to be loved, so sometimes expressing one's love towards these people and being kind towards these people may be what they need at that time.