From what I remember in April 08, is that Incheon is still one long terminal in terms of entry/exit, and all arrivals exit on the bottom/1st floor of the airport. Although there are different arrival areas (gates, etc), everyone goes through customs/immigrations before exiting, so your Mom should be able to find out which exit gate you will be exiting customs.

She could probably kill a few hours shopping or looking around on the 2nd/3rd floors, and also there is a basement level at the airport. I think there is a spa there too, which could potentially kill some time.

But in all honesty, you shouldn't have problems - the airport itself is like one huge/long terminal and you'd be bound to run into each other if all else fails after you exit customs.

Good luck!