I want to use
an elephant gun to shoot fleas.
You don't need any Arduinos for this. The XBee by itself is capable of monitoring a switch and sending data when the state changes.
The XBee on the receiving end is capable of setting a pin to match the data it received, turning the relay on.
Even the relay is probably overkill. An opto-isolator can complete the circuit, similar to pressing the power button on the outside of the case.