Wow, I can't believe that really happened. My first thought was they were pulling one over on you, but you did say you knew the black man, and it was the white mans first time there.
OK, if it had been me there as the witness, I would have said something right there to back the black man up.
My first thought was to bring white guy aside and talk to him, but if he really is one of those "good 'ol boys", you ain't gonna change his prejudice.
So after I backed the black man up in the washroom, I would have taken him aside after, and suggested we report this together to management.