Two things from a newbie. Based strictly off of the video....
when the helper gives the bite have him angle the sleeve a bit down toward the dog.
Also, when the dog bites, in the video it appears that the helper just kept staring at the dog, and stared at him all of the way out of the gate. The most simple, but maybe important??, concept I have learned is how important it is for the helper to act as if the dog affected him. Act scared, turn away, glance down, grunt.... whatever.
But do not continually be challenging the dog after the dog has shown power through the bark or the bite.