Regardless of the underlying cause, like myocardial infarction or pulmonary embolism, it must have been a cardiac arrest there. That would have required the instant CPR, technically impossible to perform in the cockpit of a small plane and certainly not by the FO. Only the immediately started CPR could have saved the Captain's life. As it happened on approach to the airport, letting him rest in sitting position with no active CPR didn't give any chances to survive even if just few minutes were required to get the paramedics aboard. That's, of course, from the medical point of view, easier said than done. Anyway, the only chance when cardiac arrest happens, is to get the Captain out of the cockpit, put him on the cabin's floor and start CPR/chest compressions by anybody capable of doing so. Then defibrillate, where the device is available.
Indeed, the cardiac arrest in the cockpit on approach is a true nightmare, happening in the worst possible moment.