"185" indicates it's a 5.0l i.e. not LS1 so that's a start.
The tach can be fixed. There's a how to on that on here - it requires moving a resistor and a wire link on the instrument cluster circuit board.
Your brother should be able to find terminal C12 at the PCM (engine control computer). That's the speed signal from the PCM to the instruments. With that backprobed (your brother will know what that means) to earth, you should be able to see a frequency output (measured with a digital multimeter) when the car is driven. If not, the PCM is probably not getting a speed signal and you will have to go to the trans. end. If there is a signal, the fault is in the cluster.
Most likely it will be at the trans. end. Again at the PCM, there should be a frequency signal across terminals F5 and F6 (when backprobed) when the car is driven. If not, either the sensor is not connected to the PCM or the sensor is not working.
Even with the sensor working, the speedo may be inaccurate as I'm not sure that the VT manual 6-speed speed sensor output matches a VS auto. PCM (it might). If not you can use one of the Jaycar speedo corrector kits between the sensor and PCM or have the memcal ("chip") in the VS PCM rewritten to suit.