1. His sleeping habits have changed.
2. He's in Pain
3. He's Gaining Weight
4. He is glued to his electronics.
5. He's having trouble making up his mind.
6. He loses his temper easily.
I'll talk with him about it and encourage him to go to someone for help. I'll let him know that I care about him and I want him to be feeling better.
If he doesn't listen to me, then I'll talk to his family about my concerns. Most of all, I'll let my friend know that depression is nothing to be embarrassed about. It's not a sign of weakness or a sign of a defective brain. It's just a symptom of something, like the sniffles is a symptom of a cold. Sometimes you need to have those symptoms checked out by a doctor to find out what's going on, and that's what I hope he'll do.