Your friend opened up a flight instrument? I think you'll find that far exceeds his privileges. It would certainly exceed mine as a licenced engineer. Instrument inspection and repair requires specialised test procedures and equipment. The airworthiness certificate may well be invalidated by unauthorised work. How does he know the case isn't leaking when it's reinstalled in the aircraft? What if it fails when he needs it most?
I'd suggest the aircraft and instrument be passed to an AMO for proper rectification; the gyro horizon will most likely then be sent to a specialised repair shop.
To answer your question, aluminium alloy corrosion tends to be white or grey.
Edit: Ah, happybiker beat me to it.