Yes, you do tell them. I mean.. why wouldn't you? If you really wanted to do these people a
favor you'd tell them, but they probably won't
listen to you... ro they may kill you; you'll just have to take your chances.
Seriously though, people are used to following false religions. How do we know christianity isn't just a bunch of bullshit some guy thought up while drinking in a tavern one night? We don't.. yet it has a huge amount of followers. There are hundreds of religions in this world, but I guess it's said that only one religion is the right one. That would make all the other's 'just made up', right? UH OH!
The fact is that millions, if not billions of people are following false religions. Do we bash what all these people hold dear? Yep! I mean, why not? They won't listen anyway.