This is another one of those questions that I am asked every time I do an Ethics workshop at a show. It makes me wonder if people are trying to be controversial or something.
Of course you would save a person in danger of being run over by a train, if you could.
We can't ever know all about someone else's path. Most of us are pretty hard pressed to know all about our own! The point is to minimize the harm, and help everyone else have fun and learn stuff. The potential harm of letting someone die by doing nothing, versus forcing them to live when that may not be the right thing for them, is negligible. If it is really someone's time to die, they will. You don't have to worry about stopping them.
Let me tell you a story. When I was a child, my parents were friends with a man who was a famous heart surgeon. He saved people's lives every day. But he told me once that when it was time for them to die, nothing he did would matter. He explained that he had performed surgery on a man who's heart was failing fast. He cleaned it out, and fixed it up, and saved the man. The surgery was a complete success. The man recovered, and felt better than he had in years. He went home, and felt so good he decided to do some woodworking in his home shop. As he was cutting a board it slipped, shooting off a splinter that went through his eye into his brain and killed him instantly. Freak accident. One in a million chance. But it was his time. He had learned all he needed to, and had to go, so he went.
So you don't need to worry. If it's time for someone to go, they will.
Let me make a suggestion. The questions like this are big questions; but how often will you really need to make that decision?
I suggest that you spend more time thinking about the little decisions that you have to make every day. Things like, "Should you tell the check-out girl that she gave you more change than she was supposed to?" or "Is is OK to make fun of someone because you don't agree with the way they dress?" or "Is is alright to flame someone if they flamed you first?" That is the kind of stuff you need to think about, because those are the kinds of decisions your life is made of.
Visualize yourself in that position, and try to figure out how it would make you feel inside if you did one thing, and then the other. It's not really that hard. If you can't tell, then try to figure out how other people would react if you did one, and then the other. See, you can do it.
The hard part is not deciding which is the best choice. The hard part is choosing to do the best thing, when it would be easier, or get you something you want, to do the other, or when you are so emotionally involved that you want to hurt someone, and bother the consequences! That is when you need to ground and center, and take the path you know you should take.
And every time you do, you will like yourself a bit better for it. I promise.