It is perfectly normal.
At first there is often just shock and fear.
Then comes anger.
The typical stages of grief include denial, anger, bargaining, depression, and acceptance.
Everybody works through them at their own pace.
As for what you can do, let her be your guide. As I mentioned, anger is a part of it all. Do a Google search on "Stages of grief" and you will find info that describes the phases.
Keep in mind that you are also going through the process of grieving. Everyone is different and sometimes people just need to be left to their own devices to work through things.
Sometimes people with cancer also get guilt feelings. "I must have dome something to cause this", "It's my fault I won't be able to enjoy more time with my family, grandkids, etc.".
Reassurance from others is nice but not always welcome. You have to play it by ear. Bad things happen to good people all the time. Sometimes stuff just happens.
It takes a while for some people to come to the realization that "It is what it is so I just have to make the best of everything".
You might point her to the site here if she is so inclined. It can be helpful talking with folks that have "been there, done that".
Mike