This sucks. Bad. I'm sorry to hear about this news, and I have total empathy.
My mother has been informed recently that her cancer has returned....for a 4th time. The first time was in 1997 when they found a beachball-sized tumor in her abdomen (she's not a little lady, so no one really noticed) which was found to be malignant. She endured a horrible and lengthy surgery and many months of chemotherapy with all the side-effects that came with it, to finally be told she won her battle. We were overjoyed. Then in 2002 I think, it came back. This time they threatened to remove her bladder, but found a specialist in another city who was able to perform a surgery that prevented that from being necessary, winning this battle as well. Then, in 2014 after my father passed, we received news that it had returned again. I feel like my mom has been underplaying the severity of her prognosis as you mentioned your mother was doing, and she said that with the treatment they'd given her that she had beaten it again. Here we are now in 2017, and she said she'd been informed that the cell count had spiked once again, indicating yet another return. It's unbearable to watch someone so close go through this even once, let alone 4 times.