This is known as sudden cardiac death syndrome, and it's basically a severe heart attack due to an undiagnosed heart problem. Mark vivien Foe, the Manchester city player, died of this during the Africa cup of nation a few years ago, as mentioned above.
Modern academies have screening for this to try and pick up the problem early, ever since Terry Yoruths son (Gabby Yoruth, the sky sports presenters, sister) died at the lilleshall football academy around 15 years ago.
I have one patient who survived this. He now has a pacemaker fitted. And he survived because my student I was teaching at the time was playing for the same team. He did CPR while the ambulance arrived and kept him alive. Legend. (The student is now physio for Burnley BTW)
Considering the quickness of treatment and the reports of him being stable, I now expect him to pull through. I also expect him to have a special pacemaker called an implantable defib (or ICD) fitted that will monitor his heart and give him a "jolt" if it fires abnormally. He almost certainly will never play again.
My thoughts are with him and his family. It's gonna be a tough few months.