According to the video, Garner wasn't in any hold when he said he couldn't breathe. He was breathing when he was put into the ambulance. In seeing the video the events are a little more clear. The arresting officer was diminutive next to Garner. Clearly Garner thought this little guy wasn't going to tell him what to do. Nevertheless the cop put Garner in a choke hold for 15 seconds. The hold had long ended. Garner was in cuffs when he said he couldn't breathe. The officers rolled him to his side and he was breathing. He was put into the ambulance breathing. Then he had a heart attack.
Absolutely the officer was not at fault.