Well, if you inspected the PA setup yourself and it was hooked up as described, that changes things.
If the mixer had pan control and left and right outputs, it is stereo. If each output goes directly to its own powered speaker, then the system is stereo. However, the sound guy said it's mono. Something is missing - either the sound guy is wrong, or there is something plugged in between that makes it mono. Graphic EQ for the room or external limiters perhaps?
Do you know the model mixer being used?