I can see why someone would get mad. Not that I am, but you are speculating and saying that something might be true because of a hypothetical scenario. Conjecture is pointless. If the person you know who believes there is a cure for cancer can't explain a good reason why, then they aren't any more informed than you are for believing there isn't a cure because of x, y, and z which are not real reasons.

And there is a cure for cancer. Cancer is cell mutation. Berries are free radical scavengers and will fight it. Avoid fructose which causes cancer cells to multiply. Avoid GMOs, food with hormones or antibiotics in it, anything not natural that would cause your cells to mutate. Consume healthy things to build good cells, and reduce exposure to carcinogens, and your body will repair itself. You don't need a magic pill from some pharmaceutical company banking on you not being cured so you have to keep buying their medicine. And you certainly don't need radiation which kills all cells and causes cancer.

That doesn't mean that people who've been exposed to vast amounts of radiation can be cured, but cancer isn't a death sentence if you control your intake and people beat it all the time.

Critical thinking doesn't mean thinking something contrary or uncommon. It means having a reason to think what you think, or else finding one. If you don't have reasons, you shouldn't form opinions. And yes, changing your mind is not only OK but encouraged as you become more educated about something. Perceptions and opinions must be adaptable or they are doomed to be the result of not enough information.