aa777888 answers the question well... in a medical emergency aboard an airliner, there is a large variability on the skills of those who may be in a position to help. EMTs, paramedics, RNs, MDs, fire/police... I am a gastroenterologist, and although I regularly take an ACLS retraining course, I would defer to an active critical care physician or critical care nurse in an emergency. But keep in mind that the current automatic external defribrillators are just that... automatic. They will tell you when and if a shock is warranted.
In this situation, we did not need to have an MD admitted to the flight deck. If the pilot has shown any signs of life or had the event been witnessed, the best course of action would have been to ask for help to move the man to the front exit area and work on resuscitation with CPR (if needed), defribrillation as needed, and whatever appropriate meds are currently in the medical kit.
This type of medical emergency happens relatively frequently with passengers. Again, the fact (as far as we know) that neither the FO or the assisting FA called for further medical aid from passengers means to me that they already felt it was too late.