KAIT123, I am also walking beside a very dear friend through the last chapter of her life. She was diagnosed with Stage 4 lung cancer which has now spread to other parts of her body. I think the loss of a friend is not recognized in our world as a significant loss. If a close family member dies we are allowed time off work and the world at least briefly treats us with compassion. It is not recognized that a best friend is like a sister, sometimes better than a blood sister because we have chosen our friend and she has chosen us to be confidant, fun sharer, secret keeper and safe support person. A best friend is someone who 'gets us', and the loss of that person is profound. I understand and share your grief.
I think Katherine's suggestions make sense: to tell your friend how much you want to be there for her and ask her what she needs: to tell her how you feel, for example sharing that you're scared, thus likely expressing the feeling she is fighting not to acknowledge. Or perhaps chooseing your moment to say 'But what if the doctors are right? Can we talk about a 'what if' plan so I know what your wishes are just in case?' I have another friend who, like yours, finds it very hard to express the 'dark emotions'; her son died a few months ago and I found the best strategy with her was to be silent and wait. This worked best if we were doing something together like a jigsaw puzzle for example, but silence seemed to work when probing just provoked a defensive reaction. You know your friend well so I believe you will find a way.
Then it will be tearful but so much easier. My dying friend and I have written her obituary together (many tears!), planned her memorial service and talked openly of our fears, sorrows and love for one another. These have been some of the most painful but also the most beautiful moments in my life. Your friend is blessed to have you at her side. Sending you a virtual hug.
Mert