I am so sorry YankeeGirl, my advice as someone who went through 10 years plus of the Big C is simply to be a good friend and to listen. Be there for her .
Cancer is terrifying at the best of times but being alone would make it so, so much worse. Support from loved ones is crucial IMO . It might not help her physically but it will make her struggle seem so much easier to bear.
Advice is usually never a good idea in such serious circumstances, I genuinely believe a shoulder to cry on and someone to talk to is so important. Be the best friend you can be. Human contact is something we all hold on to in times of stormy seas.
Take your cue from her, at this time she comes first in terms of interaction. If she wants to talk about it and opens up about her illness, treatment etc... then do not avoid the subject because there is nothing worse than feeling as though you are making others uncomfortable. Pain, death and sickness makes people often shy away from being open about those because we feel somehow it will make things worse for the sufferer ( and often selfishly because it is simply not a pleasant topic and can make us feel "contaminated")but if she needs to talk about it then it is important she does.
HOWEVER some people do not want to talk about it ( I spent most of my time with Leukaemia trying not to burden others with it and a big burden it became but I also in a way chose to ignore the illness as though it would somehow vanquish it) and that's OK too. She will show you if you look out for the signals.
Cancer often destroys the psyche as well as the body which is why remaining strong is so important and this is so much easily achieved with having solid friends and loved ones who will take a little of the burden of fragilised shoulders. Being ill often takes away everything of who you are as an individual, you lose you physical strength , your ability to cope, it makes you grieve for your old healthy self.
It can also make you feel incredibly isolated , even if I am totally honest , when you have support.
In many ways it is something you have to go through alone as nobody else can ever comprehend how it affects YOU. We all react in completely different ways , have different tolerance levels to pain and anxiety.
I would simply be available and ensure that she knows you mean it when you say you are there for her. Too many people come up with platitudes not meaning it and this can cause far more damage than not offering to help. Offer help, friendship and support ONLY if you are willing to see it through. Anything else will be a betrayal of trust and add to the insult of being sick.
Talking can be a great healer, open honest friendship. And also I would add FUN. Chemiotherapy and Radiotherapy are grossly intrusive, debilitating treatments and will make life pretty unpleasant and having something to take your mind of is very useful. A good laugh and a giggle can make things a lot more bearable.
Being treated to something once in a while is something we all enjoy.
So imagine how important it is when your horizons have considerable narrowed and the light at the end of your personal tunnel is receding fast and furiously...
In many ways I think being ill makes you feel guilty as though you are a burden so I think it is crucial she realises she is not. She will always worry about her loved ones being left behind, about them having to cope with her being ill and incapacitated and nothing anyone does will stop this.
But being shown ( actions speak louder than words) that you still enjoy her company is a good step in the right direction so to speak. Hopefully her family will support her , loneliness is a terrible thing when your body ( and mind to a certain extent) breaks down.
I am, "touch wood"cured but even 12 years on I still grieve for the person I was , as I was left with many long term health problems. Cancer did rob me of some of the best years of my life and as good as being alive can be it does leave indelible traces and scars even if you recover.
I do genuinely "pray" ( as an Atheist call it more positive vibes !) that your friend beats this thing into a pulp , because I have been there and it is hell. There are few people on this planet I would wish it upon.
Be there. Be kind, be fun, listen and support. There is not much you can do for her physical well being but you can make a big difference by simply being a good loyal friend. Something we all need , in sickness or in health.