I agree with Diana. It's definitely against the law to fire someone if she has cancer.
Keep in mind, she might be scared in general and just not voicing it because it's really hard to. I worried a lot about missing work, when really, I was scared I was going to die. There are probably deeper fears under this one about work. Not that you need to pry them outta her, but remind her she needs to take care of herself, that you're there to listen, and that you'll take her to anyone else she wants to talk to, including a support group.