Not specifically for those diagnosed with cancer though I've always been of the opinion that actually doing something, or anything, is better than the admittedly well intended line of "if you need any help with anything, please ask". That places the onus on them to call for help, that they initiate the actual act of goodwill.
When my late father was diagnosed with, and going through the (debilitating, for him) treatment for cancer, I ensured that things like the family house and my mum were looked after, for example doing the gardening, shopping, taking mum out for the day when my dad couldn't and so on. All without prompting, and all things I know would take weight from his mind.
You don't say who this person is in relation to yourself though if you know their friends and/or family then communicating with them and maybe coordinating help between you all will be beneficial. Likewise calling in on those who may be closer to this person than you are may also be of great help, when someone is diagnosed with anything potentially serious it's a wide circle that's drawn in.
I hope things go well for her, you certainly come across as someone who'd do well by her regardless of any advice on here.