[three14]

Complete anecdata, but a few months ago a neighbor of mine successfully resuscitated another neighbor with CPR. There was an AED present, but from what I heard, it did not recommend a shock. The person who collapsed is around 70 years old. Sometimes, it does work! There clearly are some cases where it would be crazy not to try it.

[JshWright]

CPR alone is very, very unlikely to reverse cardiac arrest. It's possible your neighbor lost consciousness, but wasn't actually without a pulse, and CPR just woke them up.