I have a question about disrupting a caslteside

I'm either wrong or my friends are wrong.  They keep telling me that if you check the opponents king then they can't castle no more, but I've never played a game where that happens.  You just block the check and then do the castle right?  I'm really stubborn unless someone shows me the FIDE rules; because I know my friends are FIDE type players they are the kind that don't care much for the rules, but I do care about the rules because it enhances the game.  Please reply and excuse me if I sound hot headed now, because I got a little angry how they responded it to me as if I never played chess. 

Your friends are wrong, you can't castle OUT of check if you are in check, but once you are no longer in check you can castle no problem.