Most commotio cordis( if that is what happened) events occur in baseball...from the baseball hitting the spot during cardiac repolarization...supposedly occurs with chest protectors also..from what I have read...young people have weaker thorax and cannot absorb a strong blow. Commotio Cordis can occur to car drivers sternal area hitting steering wheels in crash.. hence air bags in steering wheels and 3 point seat belts.
Good review of commotio cordis in Wikipedia.
Pray player survives.
A baseball chest protector, and a football version are worlds different.