Finding an easy meeting place outside from immigration and customs is easy as the arrival area/hall (lower level of terminal) is one long terminal. If you search on google for the Incheon airport, they should include diagram and you/mom can agree to meet at a door number, etc.

Killing a few hours is more problematic. The airport once you enter, get your passport stamped for outbound flights is excellent....just not sure if mom can access it from arrival hall. She could try to go to transit area that would permit her to go upstairs (departure level of terminal). If not, there are a few kiosks and stores in the departure area before immigration.

Not sure what's in basement level - again google search should turn something up. Here's the place to start: http://www.airport.kr/eng/airport/