I'm surprised no one has suggested the obvious answer - your friend accidentally ran into an anti-slash - and based on the phenomenon of matter/antimatter annihilation - the slash and anti-slash would have combined, annihilating both of them and releasing a large amount of energy. Did you see any bright flashes when the slash was out of sight or feel anything like a rushing of wind or energy?
Here's a friendly summary of the phenomenon:
http://livefromcern.web.cern.ch/live...AM-kids03.html