Regardless of whether there's a heaven or not - I think everyone knows where I stand on that issue - ultimately this is a question of whether the girl wants to undergo painful treatment to extend her life marginally or to accept death a short while earlier with a better quality of life in the meantime.
As an atheist, faced with that decision, I genuinely don't know what I'd do - I suspect I'd opt for better life rather than more life, but I don't think anyone really knows until they're there.
If there's a heaven then, presumably, the girl's going there sooner or later - I'm not sure how the faith element feeds into the picture. If they were denying treatment because they thought prayer would work then we'd have an issue, but regardless of what she thinks is happening after her death this is about her right to choose the death that works for her. So far as I can tell they are hoping that there's a heaven as they believe, as a bolster against the pain - I don't think I'm alone in thinking that's one of the main reasons the idea persists - but they aren't denying the very real situation of the disease and its implications.
It's just saddening that it should happen so young, with all the implications for a 'benevolent' deity in heaven that brings with it.
O.