Until he turned on the water.
It's still plugged.
This means it's plugged where the drain line gets bigger and exits the house or even beyond the house.
S will get the service guys at his company (yay for working at an HVAC/piping contractor) to camera the line and *fingers crossed* figure out what the fuck is going on. *toes crossed* that they'll be able to fix it without ripping up our stamped concrete in the back yard.