First off, the FADEC isn't in the aisle stand, or even the electronics bay - it's on the engine fan case - and yes the FADEC is certified not to be affected by liquids, temperatures/pressures (within certified limits - obviously there are upper temp limits that the FADEC electronics can survive), HIRF, Lightning, etc.
Talking Boeing installations here, but I have no reason to believe the A350 is any different:
The FADEC has the capability to turn the fuel ON and OFF - it's used during autostart, and on many engines (including the Trent) the overspeed protection system. HOWEVER, fuel ON needs to be enabled from the aircraft first - there is a hard wire signal from the flight deck to the engine that controls the fuel ON/OFF solenoid on the fuel control - the aircraft OFF signal overrides the FADEC ON signal. In short, OFF always wins, be it from the aircraft or the FADEC.
The last aircraft I'm familiar with that used a cable to control fuel ON/OFF was the 737-100/200. Everything since then uses a hardwired electronic signal that actuates a solenoid (or on a few engines, motor driven actuator) right on the fuel control to control ON/OFF. Once enabled ON, then the FADEC can use the fuel metering valve to control the fuel, including taking it to zero fuel flow if needed.
No first hand knowledge of the A350, but I suspect Electronics Pro pretty much nailed it - Airbus isn't using the appropriate impedance for the flight deck ON/OFF signal - and as a result it can be compromised by liquid contamination. Sloppy design.