For the irony, maybe she could have had an accident that she survived but then she is delivered this devastating news. You could also make her a super healthy person that constantly worried about her health.
You could say that cancer itself is one big metaphor because their bodies are just basically killing themselves.
A simile could be "cancer is like a big, hungry beast" and "cancer's like a rat. You don't notice when there's only a few and you're oblivious until you're completely infested".
With foreshadowing, you could like make her always complain about a sign of the kind of cancer she has. Like nosebleeds, if she had leukemia.
For an oxymoron, you could say that her death was a "cruel kindness" because she was suffering.
Personification could be like "cancer devoured her".
For an idiom, you could go the basic route where she would say that she sees the light.
