I really recommend turning off the feed lines under the sink before trying to work on the fixture.
I'd say same, but I think he fucked something up in the process and couldn't fix that. Or his repair skill too low to fix it - he's an artist, not a plumber, AFAIK. Well... I learned how to change ~220 V outlets before I learned where was main switch. It was easy - wires were solid, just keep your metls and fingers from them.
I really recommend turning off the feed lines under the sink before trying to work on the fixture.
They could have also thought they turned off the main water line to the whole house/apartment, but actually only turned off the line from the water heater. There's plenty of ways to critfail even when you think you're following procedure.