This is not always black and white, in the sense that the dying person may block any help. Some people dying may have too much anger, they may have too much grief, they may have too much self-pity, and other such things. So it's not always possible to help someone who's dying, it's not always possible to do anything beneficial. But if the person is receptive, one of the things we always encourage is to be there for them, and not to be there for yourself, to consider their physical pain, try to make them more comfortable.
Encourage them to think about good things, especially about their own good Kamma, and also if you know, specifically, of any good things that they have done, get them to remember those things. Try to help them to not think about unpleasant history, which often takes them into anger towards, maybe their cousin from 50 years ago, etc. It can take them into grief if they have thoughts that they missed a wonderful opportunity they might have had with a new job or something. So be careful in trying to help them think about good positive things, help them think about compassionate things, things they did that were good. By encouraging them in this way, you will be helping to give them a happier state of mind. And Buddhism does teach that the final thoughts at the moment of death do help create how a person is reborn. So if we can, try to help the person be more comfortable and to help them have more pleasant thoughts. It is better to have the thoughts based on reality, not just making them laugh or something, but something especially having to do with thinking of their own good Kamma, that may help them at the moment of death to die more peacefully and also to get a better rebirth.