Thanks for the replies. I did ask him to disable all other addons, but it changed nothing.
mrruben5, if the battle shout mode is on the addon also checks for (greater) blessing of might, so it can't be that - he didn't have that buff on him, either.
Thank you for the efficiency remark. I've added UNIT_ENTERING_VEHICLE and UNIT_EXITING_VEHICLE too, because otherwise they'll also fire for other raid units.
Code:
if (event == "UNIT_AURA" or event == "UNIT_ENTERING_VEHICLE" or event == "UNIT_EXITED_VEHICLE") and ... ~= "player" then return end