Is it possible he forgot to reconnect a wire connector or ground somewhere? There's a way to test the coolant level sensor (just a simple continuity test while the sensor tip is submerged).
I would trace the wires and make sure I didn't forget a connector and didn't pinch wires somewhere.
This may be a combination of things as well. Also make sure your battery is fully charged because ABS/traction systems are very sensitive to the battery voltage and will turn on the warning lights if the voltage drops below a certain level.
hope this helps!