I'm really sad to hear about your friend, and your difficulties. I had cancer (as a young man) and can say that you sound like a really good friend. I'm sure your friend really saw that.
The thing that surprised me when I was diagnosed is the way in which different friends each showed their true colors as a "friend." Some people really surprised me with extraordinary compassion and love. And sadly, like the mutual friends you mentioned, many of the people I thought would be really good friends to me during my time of cancer ended up being really unconcerned. Some were even very hurtful. You couldn't have described it better, “We had so many mutual friends but when she got sick they all dipped out like it was contagious.”
For me, as a Christian, the surprising thing was that most of the Christians I knew and went to church with were very poor friends, while the people who were not Christians really supported me. I have had an extremely hard time continuing in being a Christian, because of the extraordinary hypocrisy I experienced. (Loooooong story.)
Loneliness is a really sad fact of cancer that ought to be addressed and talked about. I was not at all prepared for it or forewarned, and it would have helped to be.
One thing I've heard that has helped, is that some people don't know how to interact with someone who has a bad disease, but care. They feel awkward and don't want to say anything offensive. I'm heartbroken for you and your friend regarding the “dipping out” of others, though.
As a cancer survivor, I think that your friend really appreciated your love and support, and she was very aware of it. I suppose my good friends during cancer stood out all the brighter because of the bad ones, like stars and darkness.
A year after I finished chemo, my best friend died in a tragic accident. At times I feel like I've never needed his friendship more than now, with what I am facing and what advice I seek. But I have also found a lot of peace in embracing the grief I feel for his loss, and remembering him. Rather than detesting that grief. I wrote a memoir about him, and it sounds like you would be just the person to write a memoir about your friend. Shortly before my friend died, he told me that when I write about my friends (on my Caring Bridge site), “you are passing on to everyone the value that they have shared with you.”