Have you checked out the Caregivers Support forum?
Have you read up on bipolar? That's a place to start.
Dealing with another person's depression in frustrating. We hurt because they hurt. Because it it a chemical thing, unless they recognize that and want to change it, there is little you can do to change the situation, Im afraid. Sometimes just 'being there' for the person is enough to get them through to better times. When I've been at serious lows, my room mate/best freind has stuck by me, sat up late or offered support even when he'd had other plans, even when I'm not making any sense. There is no end to how much I appreciate him for that.
I have to ask - are you afraid in a situation with this person? For their immediate safety, or your own? If so, you should seek outside, professional help to deal with the situation right away.
Other than that, just be there to listen, hug, and make sure they're taking care of them selves properly. This is a lot of responsability to take on, and there's support for you here too.