First, this is going to be hard on both of you and your presence is going to make her time easier. When I say that I admire you for standing by your friend, that is an understatement. I wish I could give you more than one “great question.” I worked hospice for a decade, tending many cancer patients, and my father died at home from cancer, me tending him.
Prepare yourself to be strong for her, try to not show your distress directly to her overmuch. However, do not hide it from her. My father died from cancer at home and I was with him until the end, holding his hand as he passed. I did not let myself cry near him other than once. The one time I did he stroked my hair and, from what I could tell, while this saddened him, it did not upset him. Whether or not you cry around your friend depends on her mindset.
Trying to pretend that nothing is wrong is not going to be helpful. Dwelling on it also is not helpful. Distracting your friend with tales of her life and things you and she did together would be a good way to make her end times more tolerable. If she reaches the point of confusion where she does not understand what you say, still talk to her… it does matter. She might appear completely confused but inside she understands to varying degrees. She might, indeed, not understand anything but she shall feel a kind voice.
Find a way for her to communicate back to you, say, a series of blinking for when she has deteriorated quite a bit and a, I think they are called, whiteboard and pen for when she can write. Her expression shall tell much of what she thinks and feels.
Now I am going to tell you what to expect physically.
You shall be shocked and upset by her physical deterioration. When you first see it it shall be quite distressing. Be ready for it… I cannot commend you sufficiently for having asked this question.
Expect her to become progressively more disoriented as the brain cancer takes its toll. She is going to have times of more lucidity and times of less, the overall lucidity decreasing. Eventually she may not appear to recognize you… this could actually be the case but often the patient does recognize you. She might eventually not be able to communicate in any way. Do not take this to mean that she is unaware of you.
Unless she has adequate pain control, she is probably going to be in increasing pain, requiring high doses of morphine, dilaudid, a phenergan patch, etc.
Cancer also has a smell to it, at least one that I can smell. It is usually not strong but it is noticable.
She shall most likely be incontinent of bowel and bladder, probably shall have a catheter to take her urine. I am hoping that she is kept clean but, even when a patient is kept clean, usually the smells of incontinence linger.
You say that you are afraid you shall not be strong for her. I am willing to bet that you shall be strong enough because you are intelligent and caring enough to ask this question.
If you need someone to talk to, feel free to PM me. Along with my father dying of cancer in our home I have worked hospice. One thing for you to remember that shall help you stay strong is that you are making her final days better.