My friend punched me in the chest, on the left side. he did it pretty hard too. could this cause commotio cordis? or would i be on the floor dead?

Possible, not likely: This is a very interesting question. When I looked up risk factors for this, I found "that commotio cordis events are due largely to the exquisite timing of blows during a narrow window within the repolarization phase of the cardiac cycle, 15 to 30 msec prior to the peak of the T wave." In other words, the blow's timing is more important than its force. Who knew? Hope that helps.