OP my elderly mother was diagnosed with late-stage incurable cancer a few years ago.
I don't wish at all to sound pessimistic or negative, but it all depends on how old and ill your mother is, and how far her cancer - I am so very sorry to hear about that - has progressed. I very much hope that the doctors etc can do something positive for your mother. In which case, as others have said, you should at the moment take a back- but supportive - seat, take notes, and get contact details for answering the inevitable future questions. And then be able to go through, slowly and gently, in her own time and place, all the info with your mother. I'm not saying that your mother is not intelligent or capable. But shock - and severe illness - really takes its toll.
But just in case - heaven forfend - there is not much physically they can do, can I please share my experience with you?
My mother's consultant was a whizz at medical expertise and diagnosis but- as others have suggested - psychologically pretty brutal. After saying that my mother was too old and frail to operate on and that chemotherapy would not be very effective - all, objectively speaking, true - he simply said that 'we'll have to let nature take its course'. Then he - in effect - closed the file.
The whole consultation lasted maybe 7 minutes. My mother, naturally, was totally shell-shocked.
We were then passed to his very charming nurse assistant who - in a lovely and polite and friendly way - did the paperwork to discharge my mother into the community for terminal care.
And - this is the point of my story - my mother simply could not get her head around this. She did not understand the organisational gulf that sometimes exisits between hospital diagnosis and hands-on care or support in the everyday world. To my mind, that is really, really important.
In her last months, my mother kept saying 'Why can't I see that nice nurse again?' (Such a meeting would have been of no clinical use but would undoubtedly have been psychologically supportive.) But, according to the 'system' (not criticising, just describing) the consultant and 'nice nurse' no longer had any responsibility for my mother's care. My mother's GP was excellent in organising painkilling medication and whatever what help she could send, but the community nurses and the community matron were most definitely not. My mother was 'assessed' so many times by nursing people with clipboards, but none of them ever did anything much hands-on to help. The Macmillan Nurses were involved, but sadly the local Macmillan nurse was ill herself and had not been replaced. So they visited twice in four months, I think.
All this may have been because we paid for living-in care for my mother. So she was warm and fed and comfortable and had someone to call on during the night. Very, very expensive, but it was what she wanted. Sadly, it turned out that she could not die at home. Her GP visited one day - which she did (well done her) very, very regularly - and decided that the time had come for hospice care. The hospice was a lovely place. Had my mother been willing to engage with it much earlier the people there might have helped her a lot. But she was not. It was hard to encourage her, but if the time ever comes for this for your mother, then I would encourage her to do so.
I am sorry to ramble on. Even after several years, this is obviously still raw. I think what I am saying is the biggest support that you can be is to help your mother engage - in whatever way is best for her - with all the local curative (clinical trials etc) if relevant BUT ALSO later-stage care services in a way that feels good for her. These will vary from are to area. It is often not easy to find out what's available or what would be best for any patient. You, as someone with NHS experience will be much better placed than most to do this.