She sounds like she suffers from depression.
When she is up she is way up, when she is down she is way down.
I get that way. I have a few friends I can say, I am really having a hard time right now. It has taken me until about 2 years ago to do this. It is embarrassing and frustrating for me, so I know it is even worse for them.
At some point when she is way up.. speak with her and tell her how much she means to you. tell her you wish you could help her as much as she has helped you. Let her know you feel like she may not understand exactly HOW MUCH you really do want to help at any time.
I have a friend that told me she loved me very much and really appreciated everything I do for her. She gave me very specific examples of things I had done. She then said, "I want the honor of doing some of that to you. I feel like sometimes you disappear and I get worried that you are mad at me or do not want me around>"
This helped me open up to her about how sometimes, I just shut down. That I get depressed and overwhelmed and just trying to get through the day takes every bit of the little energy I have during that time.
So depending on how close you are to your friend, do tell her sometimes, you miss her or feel like you do not get a chance to help her. But that you are always there for her and appreciate her.