@Videoguy, thanks for reaching out and for spreading the love for the Rachio controller 
The Hunter Solar Sync is a proprietary accessory for certain Hunter controllers. As such, it’s not compatible with the Rachio controller.
However, the data that the Hunter Solar Sync collects is done automagically on the Rachio controller, so the sensor isn’t needed. Does your brother need a rain sensor per a water utility requirement? If so, I’d recommend selling the Hunter controller (that Solar Sync is worth $80ish on Amazon) and purchase any of the following rain sensors that we support.
I hope this helps.
Best, Emil