Really. Really? Really! Reeeallllyyy?

My nine-year-old daughter was angsting recently on the fact that one of her school friends says her grandpa can swim across the Mississippi with his hands tied behind his back.  My daughter and I talked a bit on how one should respond to such to such claims that are most likely untrue.

I don't think we came to a consensus, and I am left with questions:  Is a socially polite answer like "Really!" or "You must admire your grandfather" make one complicit in nonsense?  Is it ok to just not say anything?  Should one resist the urge to argue against something that's not true?  Is there harm in playing along?  

And moving into larger issues, what to say when someone shares a statement on  immigrant status, national origin, sports team or religious affiliation or politics that can't be accurate or might even be harmful.  How does one respond then?  And  how can one be a safe listener to help people work through thoughts while not contributing to the growth of potentially harmful thought and speech patterns?

What are your thoughts on any of these things?  And is that a rhetorical question?