Boy can I relate to this. She was probably "afraid" she had cancer. Once she "thinks it", the cancer becomes a fact, so when the doc tells her it's not cancer, she doesn't believe it.
I would do nothing. Let it blow over. Eventually it will be obvious she doesn't have cancer because she won't be having those end of life cancer symptoms.
She will probably have other symptoms that are "real to her", and she will need someone to take her to the doctor which gives her more attention.
My mom has cried "wolf" so many times, I don't get too excited about it any more. If she doesn't get attention for it, it often goes away on it's own. My mom is 86 and likes to tell everyone about all her health problems, but she doesn't want to see the doctor. She also refuses to follow Dr orders and prescriptions.
When your mom says she's "eaten up with cancer", what she's probably really crying for is
attention. But one doesn't really want to reinforce that behavior by giving her attention for that reason. If she gets attention for that, you will see more of that behavior. Better to give unsolicited attention for something unexpected (nice hairdo today mom, or lovely flowers in your flower bed or whatever would work in your situation).
Alternatively, call her on it, and ask her if she thinks she needs to pick out a casket yet.
Sometimes humour can arrest it too, but perhaps someone else could suggest a line where humour would stop her in her tracks. I'm all out of humour at the moment.
My mom has had herself and many acquaintances dead in the ground. They are perfectly fine. BPD thoughts are always so extreme. I would let her rampage situation blow over or burn itself out. Eventually she will self-soothe.
