What will you do if your friend got depression?
Do you think a friend will be as helpful as a doctor?
what should I do if my friend refuse to see a doctor?
what should I talk about instead of just saying everything would be better?
The advice I'm about to give is easy to say and hard to do!
A friend is not the same as a doctor. A friend doesn't understand the symptoms. A friend hasn't seen hundreds of other people with the same problems. A friend cannot tell if medication will be useful and cannot prescribe medication.
But a friend helps in different ways than a doctor.
The biggest common factor in all successful psychotherapy is simply the presence of a sympathetic and nonjudgemental listener. You can help just by being there and listening. Don't tell your friend to cheer up or try to point out the positive, but do stay friends and do try to keep being there and listening.
If your friend really has clinical depression, your friend should see a doctor, but if they refuse to see a doctor there's not much you can do. Do your best not to keep nagging them about it.