It's shockingly common, too. A close family friend saw progress from an experimental treatment against stage 4 pancreatic cancer (she basically stole some eighteen months of her life back; this cancer at this stage is notorious for being a quick and agonizing death sentence), but despite success in shrinking the metastasized tumor population down to just one, she had the misfortune of her cancer mutating such that the treatment was no longer effective.
It's shockingly common, too. A close family friend saw progress from an experimental treatment against stage 4 pancreatic cancer (she basically stole some eighteen months of her life back; this cancer at this stage is notorious for being a quick and agonizing death sentence), but despite success in shrinking the metastasized tumor population down to just one, she had the misfortune of her cancer mutating such that the treatment was no longer effective.
She was felled two weeks after that discovery.
I'm so, so dreadfully sorry for your loss.