I'm sorry your friend has cancer. It is a worrying diagnosis but also thesedays it is a much more treatable one than it used to ne. My sister recently underwent surgery too and then radiation, and so far all is clear. and that was with a 4+ diagnosis, which is almost a miracle.

Dominatrix' do compartmentalise their lives and they do not take kindly to people stepping out of their compartments. Send her a friendly get well card and let her know you are concerned for her, that will be more than enough for her and if she wants or needs anything else from you she will let you know. But don't step outside of your boundaries or she'll be as pissed as hell