Unfortunately getting muzzle flashed is an all too common fact of life. I've learned to always say something to the offender. I'd rather be thought a prick than get shot. Other shooters will appreciate it too, when they see you have concern for their safety.
Never rely on range officers to do their job. Your safety is in your own hands.
Proverb for Paranoids: The innocence of the Creatures is in inverse proportion to the immorality of the Master. -T. Pynchon