It's not your responsibility. Try to remember that.

I have a friend who's depressive, and we would get in these awful downward spiral conversations. What I decided, without really saying it, is not to respond to negative things. We talk about very superficial or lighthearted things only. If the conversation starts to go south, we end it. We feed on each other too much to let it go. It sounds impossible, and it's really hard to fight the instinct to be hurt or to try to argue with someone who's depressed, but with a little practice, we got the hang of it. You could try it.