Her situation parallels my own: I was three years older than her (20) when I was diagnosed with cancer, and my chances of survival were worse than hers (it changed, especially after I relapsed, but my chances were only ever around 20%), but on the whole, I was in her position. I do think she's foolish for giving up when she has the odds of survival she has (although I don't know for a fact she doesn't have other complications). Chemo isn't nice; it affects some more than others. I was one if the people it affected badly. I threw up probably 20 times per day or more for the ten days straight of chemo that my treatment consisted of. And those ten days were just one course. One of nine courses overall (including the five I had after relapse). Then I had radiotherapy, and finally a bone marrow transplant. All of which resulted in more vomiting, ridiculous frailty and exhaustion, infections etc. And all this alongside a general resignation to what I thought would be an inevitable death. I probably spent a sum total of at least nine months in hospital in 2006 through early 2007. Yeah, it was bleak. But there was never really any question I'd fight it. And, guess what, I'm cured now.
But I could have reacted differently. I'm glad I didn't. But at that time in my life, I was already depressed and friendless. I can't deny my diagnosis was, in an effed up way, some relief to me. I just happened to have a 'roll with it' attitude to my treatment because of my feelings. Deep down, I did want to live and I always knew it. But I cultivated the apathy to help me deal with it. This was my way of dealing with it, and this girl has hers. Maybe she even feels the way I did, just more strongly. In any case, it's her choice.
I'm not American, and I don't even know whether I could have legally declined treatment. I know old people definitely can, as is probably also the case in the US. But I never actually even considered it for myself. I just thought my low odds of surviving would take care of it for me.