There comes a time where someone close to you becomes depressed. As a friend, try and talk to them, let them know that you care about them, and most importantly, NEVER give up on them. It's hard I know, but even if they show no signs of thankfulness, they really appreciate what you do for them. I too have had a friend that used to be so happy about life, but after he lost his little brother, he was a broken man. but after many months of being there for him, he began to heal.
If you feel like no matter what you do for them and they still act that way, keep trying! don't let them lose hope. One friend has the power to save a life, even if the other has lost hope. Be the shoulder to cry on, be the ear that listens, find a way to make them happy again. Never lose hope.
And if you are someone who is struggling from depression right now, even though I may not know you, keep hopping for a better life. One day all the pain and suffering will be worth something. Keep your head held high, you are beautiful. Don't let anyone make you feel different. :)