It's such a helpless feeling to watch a friend or loved one go through things like this. While there is nothing you can say to make it better, just being there for her will help, even if you can't see that it is helping.
Sometimes she might want to talk, or cry, or rage, and other times, she might want to be silent. Follow her lead and just be there.
At some point, a distraction like a movie or some other activity would be helpful to get her away temporarily so she can recharge. Encourage her to do this. Even if she says no at first, just keep asking.
Ask her to go for walks with you. Again, she may or may not want to talk during this time. Just being outside in nature and knowing she has her dear friend beside her can be healing and provide her with a sense of support and strength.
If she has small children, offer to take them once in a while. This will be really helpful if she has to accompany her mother to medical appointments.
Encourage her to take care of herself. Many times when people are in grief and in caregiver roles, they start neglecting themselves. This is especially important, given her medical condition.
You are a true friend, and she is really lucky to have someone like you in her corner. I hope things will turn around for her and her family. I will keep them in my thoughts.
J. F.