The only hopeful news is that, in cases where defibrillation is possible within only a couple minutes, as it was here, they are able to restart the patient's heart three times out of four...and with a physically strong individual like Muamba, it's likely higher than that.
Cardiac stress testing should be mandatory for all professional players. This simply happens too often, and with an estimated 1 in 200 people having hypertrophic cardiomyopathy (which is usually the cause when young, otherwise healthy individuals suffer cardiac arrest) it's likely that there an awful lot of people in the game who have undetected heart conditions.