Why is everyone calling the student a tard? The Royal Guard are a tourist attraction, and are *known* for not being interuppted/distracted. It is the very fact that it is against their orders to be disturbed/distracted/etc. that makes them so popular.
He (the guard) will have been told, and probably knew anyway, that tourists will mimic, pull faces, wave their hands infront of their faces, take photos whilst stood next to them, and generally 'pester' them.
The Guard deserves a dressing down; if anything he has performed misconduct by not following his orders to ignore everything but a threat to the royals..