I am so sorry to hear this, I have lost my mother to cancer earlier this year. It's a pain that I could have never imagined before. I think the most important thing you can do is to be there, spend 1000% of available time with her, do whatever she wants or can bring a smile to her face, take pictures, record her voice.

Some things I have noticed to helped me:

1) Exercise - going to the gym regularly, even through mental pain, helped me a lot in the long run

2) Eating out - I started eating out more often and having nice, cooked food helped a lot

3) Working - my employer was very understanding and tbh I found it actually helped me to work, as allowed to take mind off things, though you could be different.

4) Hobby/gaming/Lego