My wife’s good friend was having back pain, she was “diagnosed” with cancer of the spine. She was put on chemo and died two weeks later. An autopsy was performed and it was found that she didn’t have cancer at all. Some silicone had leaked from her breast implants and settled near her spine. It most likely had nothing to do with her back pain, but caused her x-rays to look like cancer had metastasized to her spine. Apparently some of her blood work came back with unusual results as well, but there was no cancer found in her body. The chemo was determined to have killed her.
There sounds like there are missing pieces to that puzzle. An oncologist would not do chemo without verifying cancer. Not all cancer is in tumor form. Perhaps she did pass from chemo, but it sounds like some information is missing from her diagnosis.