How to help a friend?

If she's truly bipolar, she needs to be diagnosed and treated professionally. Amateurs aren't qualified to make recommendations. Even if she has mood swings that make it difficult to get along with her, that doesn't necessarily mean she's bipolar.

I personally would recommend Zoloft. It is the only thing that worked for me. Paxil was like the devil in my head. Horrible pill there in my opinion. I was diagnosed as manic depressive and also social anxiety disorder and Zoloft is the only thing I have found that makes me half assed calm and collective. In the meantime all you can do is be supportive and just make yourself available as a friend and try to show you do care. This type of problem is never fun to deal with, but it is better when you have friends and family who care for you.

You don't mention if your friend is under a doctor's care. She should be if she isn't. It will help even her mood swings out to be properly treated. Either way, when you see her starting her downward swing (especially in public), then it's time to take her home and leave her there. You need to take care of yourself, including protecting yourself from her difficult side. Otherwise you're libel to burn out and won't be any help to her at all. I'm not saying you should abandon her every time she's in a bad mood, but you don't need to put yourself constantly in the way of it either! It's her chemistry, not her personality that's the problem. You are not responsible for anything more than being sure she's getting the professional help she needs. God bless you for being such a caring friend!