Perhaps just being there for you friend can help get them through. Remind your friend of the small simple, yet wonderful things about life. Do they have any hobbies? Places they love to be? Take walks with them, talk about life, find the source of what's bringing them down to such a state of mind. A reminder of the beauty of life can surely help lift the spirits, but of course, the severity of their depression is important to consider.
Though, again depending on the severity, profession counseling may be the best option. My girlfriend has began to seek professional help for her PTSD, and as much as I tried to help her, her condition was better left in the hands of a professional.