Everybody should get some CPR training, with regular refreshers. You never know when someone is going to go down at the supermarket, in church, at the movies, in a parking lot, or at a baseball game. Be the person who knows what to do.
In the tension of the moment, it's hard to think straight unless you are in an ER with support and help. And people collapse all the time, usually for minor and relatively benign reasons. But sometimes it's an arrhythmia, or a heart attack-related arrhythmia, and, if so, it's your chance to try to save a life as long as you aren't too far from definitive help. In the wilderness, forget it. They're a goner if they quit breathing. CPR is a temporary thing.
Online CPR certification here. (with sound). It's best to practice it with a dummy, though. I have only had to perform this once in a non-medical setting. A rainy, cold, wet parking lot. She survived, but with mild brain damage from hypoxia (the cause of her collapse and subsequent respiratory failure was a ventricular arrhythmia of unknown cause). And I cracked a rib or two, but that hardly matters when someone is "trying to die," as docs call it.