How could one (and should one) convert someone from pseudoscience?

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?