So sorry your friend faces this diagnosis. You are wise to consider options for reaching out to her. Here are some tips:
Don't try to make plans to help out or just be with her either too far in advance or too quickly. Sometimes a mid morning call that you would like to drop by at 2pm with a specific time to leave, or an evening call that you would like to bring lunch the next day could be the kindest thing ever.
Offer to read an encouraging book through with her, something like The Incomparable Christ, by J.O. Sanders, http://www.goodreads.com/book/show/653151.The_Inco... . This may give an opportunity to have regular visits during which you can offer to spend more time with her and/or have the chance to see if there are unmet needs you could to step up to bat on.
Ask her if she wants you to research any aspect of her condition for her--various treatments, treatment centers across the country, how to talk to doctors about her condition, what to expect (rather than just taking the word of a person she is currently seeing), etc. Maybe even offer to travel with her if her best option is out of town.
Offer to take her to appointments as her advocate during which you would take careful notes or record what she is told. Ask her ahead of time if she wants you to ask questions during these visits and be ready to respond to docs, nurses, etc. who may be intimidated.
Tell her that if she is overwhelmed by family or friends who want to enter her life at this point that she can use you as a buffer. You could be her appointment secretary or coordinator so she can periodically turn her phone off if she needs to during treatments.
Initially her husband/child(depending on ages)/parent may be doing much of the above, but as time goes by they will have to keep their job, go back to school, or any number of things. To know that she has an advocate in you if she needs one may be a huge help and comfort to her.