I've had some pretty low points in my life so far. I don't know what it is, but suicide or cutting myself never entered my mind. It was pretty bad stuff too, I used to get bullied basically every minute of every day at school and I never told anyone about it?
Know how I got over it? I told people, they listed, gave me advice and I listened. I kept thinking "This will stop, it has to stop soon" - and it did! Now, I enjoy school and things are going well for me.
The moral? Tell "your friend" to try and look on the bright side of whatever it is that is causing his/her depression. Always think of this bright side, never let it of your mind. Life is hard, we all know that, but remember, bad points only last a small while of your, hopefully, long life.
Pyschiatrists can also help. The main fear is that they will "laugh at you". Don't think this. It's their job to listen and help, not to mock. That's what they get paid to do. Tell them, I'm sure they'll have had lots of experience and will be able to offer suitable advice.
Could you tell us what causes "your friend's" depression?