How do you accept that someone you love won't be around much longer?

One of my best friends has cancer and it's so horrible to the point where she doesn't want to live anymore. She knows she doesn't have much time to live because she was diagnosed pretty late. She wants to go through with euthanasia (physician isted suicide). I don't want her to go through with it but she said she wants to because it's so horrible and unbearable to live with cancer. I've never had it so I guess I wouldn't understand. How do I accept that she wants this. I'm finding it so difficult. I don't want her to go :(