She knew how serious her condition was and yet insisted on going on a camping trip. Might it be that she made a bargain with a creature? Not for her benefit, but rather for yours. I'd suggest looking up more about where she went, whether there were her friends with her or she was alone. The first step would be to check her location history and investigate for any signs of rituals there. Because tumours don't just go away, you don't just get a second chance at life, at least, not without a cost.