An AED will only advise the rescuer to deliver a shock if the heart is in a rhythm which can be corrected by defibrillation. They are also made to include voice and visual prompts. For this reason, AEDs are easy and safe to use.
An AED can be used by anyone, regardless if they have been trained on them or not. However, it is highly recommended that everyone get certified in CPR and AED usage. CPR and AED training and familiarity will increase confidence in the rescuer which will lead to more efficient CPR and a more timely AED application and usage.