I'm Type II so probably not going to be much help here. My depression is horrible, though my mania is never really high.
The advice I can give you is something like...I don't know.
In my experience when I'm depressed I am a downer to be around. I say all of these horrible things about myself and melancholy observations about the world. My poor friends don't know what to do when I hit rock bottom.
Try talking to her when it happens, try to identify signs and symptoms and such. It's helped me a little by doing that with a close friend. She would ask me questions like, "How's your mood been? When did it start? What do you think caused it? Etc."
And I would reply and work through it.