Do Not Ressusitate we had one done for my mother cpr can be brutal and we agreed not to do that to her.
A DNR would have stopped them from even starting CPR - not cause them to stop halfway through (though, I suppose if someone brought them one while they were doing CPR, it might cause them them to stop). I agree though that it is unlikely that she had one.
There are a number of rules used for when to stop CPR - basically ones that show it is clear that the patient is not going to be saved. I know that there were rules for me when I was lifeguarding, but I'm sure that they are different for the EMTs (they have more medical knowledge and they have special equipment).