The original question, in Message #1, was one of how others would react in the scenario described. The scenario was that a terminally ill person in severe pain without autonomy requesting his life be ended to escape his torment.
These matters are best managed in the moment. One deals with the problem at hand.
Your friend feels scared, hopeless and powerless.
If you wish to help, hear him out, share with him, his suffering.
Being there with him, he will know he is not alone and that he is loved.
He may wish to talk about fears, anger and disappointments, which can be very difficult to hear.
If you can get through it, you will certainly find it a benefit to yourself as well as to your friend.
With experience in these matters it becomes clear that a basic principle of oncology is that people should not needlessly suffer.
One of the side-effects of pain medication is that it may, not by much, hasten a death that is inevitable.
To give an overdose, i.e. more than what the person needs to control the pain would be in most cases considered malpractice.
Some people are given the grace to live the experience of their last moments to its fullest: to be with Christ on the Cross. Usually we are unconscious and simply and slowly drift away.
All this temporal suffering ends and we are reborn to the fullness of eternal life.