I've only got basic first aid training but I'm very surprised that 6 minutes would be enough to declare someone dead. I've always been taught that if you are tending someone who's not breathing or heart has stopped you don't stop giving CPR until help arrives. Even a doctor in that situation should keep giving CPR until someone with better equipment can take over.
Once the ambulance arrived I assume Tomlinson was defibrillated before being declared dead? 6 minutes between the ambulance being called while he's still conscious, him stopping breathing and CPR being administered until the ambulance arrived, the paramedics arriving and attempting treatment before stating he is dead seems very fast to me. I could be completely wrong, and I only have very, very basic training, so perhaps that is normal.
Either way I doubt there was anyone immediately available within the police who were there who was more qualified than a 3rd year medical student. I hope stopping her from continuing to attend him until the paramedics or a doctor arrived will be seen as criminal.