Grumpy_Mike:
No problem with the relay but it could be a problem for the monitoring arduino.
Is your relay 5V? Has it got a reverse polarity flyback diode across it?
I'll check for that diode, wouldn't have thought of that, I'd need to pick up the output from the MCU to the relay from before the diode (at the existing MCU possibly).
It almost certainly will be at or less than 5V - PIC MCU controls it.