Regarding how to test the relay I would suggest that your friend first determine with the multimeter that the water pump is or is not getting +12vdc....and that the motor wire connections are good. It could be a dead motor too. If there is no +12vdc at the motor then you can start checking the pump relay (once you find it). Easiest verification of the pump relay is turn on the pump power switch (red light comes on) and have the other person listen at the pump relay for the pump relay to click (operate). It might be a very low sound. If pump relay does not operate check for 12vdc and ground on the relay coil connections. If the relay operates OK then check for 12vdc on the relay control contacts which when operated, connects the 12vdc through to the water pump motor. It could also be loose wiring connections somewhere in the pump/relay circuit.
Attaching a general video on how a relay operates.
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