Remember that a little bit of rationality in gullible hands can have unpleasant consequences. Next thing you know, they'll be the ones accusing you of being unreasonable.

There is a range of possible courses of action open to you. On one end, you can happily live with your friends' strange ideas and not interfere at all, even when they teach the same ideas to other people or to their children. On the other end, you can make new friends whose ideas are closer to yours and let your old friends fade from your life. My personal experience has been a case-by-case mix of both approaches.