AEDs are safe to use by anyone who has been shown how to use them. The AED's voice guides the rescuer through the steps involved in saving someone; for example, "apply pads to patient's bare chest" (the pads themselves have pictures of where they should be placed) and "press red shock button." Furthermore, safeguards have been designed into the unit precisely so that non-medical responders can't use the AED to shock someone who doesn't need a shock.
It is unlikely. Studies show that AEDs interpret the victim's heart rhythm more quickly and accurately than many trained emergency professionals. If the AED determines that no shock is needed, it will not allow a shock to be given.