I like to read these threads to remind myself to never associate my spirituality with any common adjective. Yeah, the bible thumpers are annoying, but the agnostic and atheists are getting just as unpleasant.
Maybe instead of asking a moronic question on the internet, you can try to empathize with your friend who might very well lose her father. In my experience, cancer does not lend itself well to common sense, and you just have to try to support those who are going through it.
Just be there for your friend, even if her father's cancer does not end up being terminal, it will still likely shake up her beliefs.
Now don't get me wrong, faith has interfered with proper medical treatment before, and am not saying that prayer supplants the amazing strides we have made in medical science. This is more for the emotional side, and it is pointless to get into a theistic debate with someone who is dealing with cancer on any kind of level.