I am sorry to hear about your friend. Aspiration and choking are well-recognised causes of low oxygen and obstruction of the breathing tubes causing death. Some of these are preventable if the airway can be cleared of the foreign body (like food) by first aid techniques like the Heimlich maneuver. Having a weak cough is probably why the food could not be coughed out which is a normal protective reflex. If the oxygen is low, the heart also gets affected and the blood pressure can go down and fainting occurs.