If someone who ought to do better falls for the 'if you have nothing to hide' nonsense...

You could ask them to imagine having innocent things to hide, such as being trans or lesbian or bi or gay and having to hide this from homophobic parents and/or bosses, or being a survivor and having to hide this from anyone who might be a compulsive victim-blamer, or being an activist [though all too many people think concern for humanity is treachery to any given country].