This is not a black and white question, with a fixed answer. It depends a lot on the relative's background, and it depends a lot on how spiritual they are.
People who are spiritual, usually have a concept that something is coming next whether it is heaven or the Buddhist concepts of another birth. People who have a concept of something coming next can be helped much easier. Encourage the person to think of their goodness. Encourage them to think of all the good things they have done in their life, and if they are able, even do some more generosity deeds before they die so that they can think really nice, positive thoughts about themselves.
This could give them the conditions so that at the moment of death, their thoughts will be happy thoughts, thinking of goodness, which then propels good Kamma to fruit in the next life. If, at the moment of death, we are thinking a lot of negative thoughts, very heavy thoughts, then that helps our bad Kamma to fruit first in our next birth. If we think positive thoughts it helps our good Kamma to fruit first. So if a person is able to be encouraged to think of their good Kamma, to think of their good generosity and so on, then that can help them develop more positive thoughts while they are dying.
If they are not a spiritual person, then it will probably be harder to do this. But you could still try to encourage them to think of happy memories so they don't get afraid or angry or into negative thoughts.