I'm not bipolar, but I am no stranger to depression either. Words fit well in their place, however action is the it for me. "Hey, I'm going to handle some business come along and keep me company." Do lunch and the whole nine yards full of light, normal conversation. Bring your friend back to the light of day, to the joyous... reality of not being depressed. Perhaps while on the ride along with you, your friend can find himself/herself again. If this doesn't work one day, then give it another.