While happy thoughts seem exceedingly unlikely to by themselves cure any sort of case of cancer, certainly persistent unhappiness can lead to many physiological changes which are associated with degenerative disease and cancer risk (stress, trouble sleeping, and so on).
As Lumifer pointed out, it's important to consider what the practical consequences are of their beliefs. If the person you're referring to simply believes that engendering a sustainably happy state of mind will decrease their cancer risk, then I doubt there's anything to worry about. But if you would expect them to refuse a proven surgical technique and instead attempt to cure themselves by hanging out with their friends and watching fun movies, then surely it would be a highly useful service to this individual to point them in a better ideological direction.
Don't introduce them to a catalog of logical fallacies. Understanding a few important logical fallacies can help people who possess a propensity to propagate new conceptions through their web of beliefs, figuring out which beliefs should stay and which should go based on their new theory. But most people don't operate in this way. Updating all the different areas of one's belief structure in accordance to a newly acquired abstract tool doesn't come naturally to most individuals. If you coax a friend down the path toward Less-Wrong-style rationality, then there may come a day where reading 37 Ways Words Can Be Wrong would be quite an enlightening experience for them. But that day is probably not today.
I wonder whether your impression that the world of pseudo-science is a rosy one, and rationality is a window into the bleak reality of human life, is the key to the frustration you're communicating here. The only language that your non-rationalist friends will appreciate is the language of concrete results. If you can't employ your ability to think rationally to become noticeably better than them at activities they pursue in a serious way, give them health advice which to them seems to miraculously clear up certain long-standing inconveniences in their life, etc., then you're not giving them any evidence that your way of thinking is better than theirs.
Use your capacity for rational thinking to succeed in concrete endeavors, and then demonstrate to them the results of your competence. One day they may ask to look under the hood--to see the source of your impressive abilities. And then the time will have come to introduce them to the abstract rationality concepts you consider important.
Stress and having trouble sleeping causes cancer?
I've known for a long time that some people who are very close to me are somewhat inclined to believe the pseudoscience world, but it always seemed pretty benign. In their everyday lives they're pretty normal people and don't do any crazy things, so this was a topic I mostly avoided and left it at that. After all - they seemed to find psychological value in it. A sense of control over their own lives, a sense of purpose, etc.
Recently I found out however that at least one of them seriously believes Bruce Lipton, who in essence preaches that happy thoughts cure cancer. Now I'm starting to get worried...
Thus I'm wondering - what can I do about it? This is in essence a religious question. They believe this stuff with just anecdotal proof. How do I disprove it without sounding like "Your religion is wrong, convert to my religion, it's right"? Pseudoscientists are pretty good at weaving a web of lies that sound quite logical and true.
The one thing I've come up with is to somehow introduce them to classical logical fallacies. That at least doesn't directly conflict with their beliefs. But beyond that I have no idea.
And perhaps more important is the question - should I do anything about it? The pseudoscientific world is a rosy one. You're in control of your life and your body, you control random events, and most importantly - if you do everything right, it'll all be OK. Even if I succeed in crushing that illusion, I have nothing to put in its place. I'm worried that revealing just how truly bleak the reality is might devastate them. They seem to be drawing a lot of their happiness from these pseudoscientific beliefs, either directly or indirectly.
And anyway, more likely that I won't succeed but just ruin my (healthy) relationship with them. Maybe it's best just not to interfere at all? Even if they end up hurting themselves, well... it was their choice. Of course, that also means that I'll be standing idly by and allowing bullshit to propagate, which is kinda not a very good thing. However right now they are not very pushy about their beliefs, and only talk about them if the topic comes up naturally, so I guess it's not that bad.
Any thoughts?