You would get the foci discount on just the device, but the Sidekick power itself could be Attached tot he magical device, and then the powers the Sidekick has would be Attached to the Sidekick.
Magical Device (1/d)
Useful (Range). Focus (Accessible, Immutable) -3, Duration +2.
Sidekick (3/d)
Defends (Self). If/Then (Only usable by Sidekick) -1.
Useful (Range). Duration +2, Attached (Magical Device) -2.
Able to Affect the World (1/d)
Attacks (Mass). If/Then (Only usable by Sidekick) -1.
Has wound boxes (2/d)
Useful (Self). Engulf +2, Endless +3, Self Only -3, Attached to Sidekick -2.