it depends what shes depressed about. like if its a breakup say ull find someone much better, if its a death in the family or among her friends, say they r in a better place now and they r looking over you.
Listen to her, listen to her, listen to her... and don't talk. You really don't know how she feels. Finally, if and only if given a chance to speak. Choose your words carefully. Like, it's safe to say "Let me know what I can do for you" and really mean it.