Probably I'm not alone in such situation when a good person you know believes in something really harmless but still ridiculous in the depth of your mind.

For example, that astrological sign seriously defines compatibility of people, or WTC was demolished intentionally in the US, or GMO is bad because in some experiment rats reportedly stopped reproducing and therefore someone wants to shrink certain human populations by popularizing GMO.

None of such beliefs actually do something directly harmful but sometimes just the fact people you know believe that is almost disgusting. Any proof would be discarded as an "official", biased one, profitable for those who have money and power.

What should be done in such a situation? What would you do if your otherwise perfect friend seriously considered Earth flat?