There is no way anyone should attempt a dive of that depth with air tanks so they must have been using mixed gas. I retired my PADI Instructor license long before that technology was readily available so I don’t know either.
He was hoping to video a sawfish, said to be a skittish critter likely to be scared away by divers' bubbles. So his plan was to use a rebreather (no bubbles). In December, he posted to Facebook “looking for a rebreather dive ninja mid January in Florida for some 300ft dives…”. That's how he hooked up with survivor and defendant Peter Sotis, of Add Helium.
According to this article:
Stewart and Sotis dove the [Queen of Nassau] wreck three times that day. They were the deepest dives of Stewart’s life. The two men surfaced for the final time just after 5 p.m. within sight of the dive boat’s crew members. Stewart gave the OK sign. Sotis, however, appeared shaky as he climbed aboard the boat. Moments later, he blacked out. The crew retrieved bottled oxygen to revive him. In the commotion, they turned their backs to the water, and when they again looked for Stewart in the water, he was gone. The crew radioed for help immediately. Within five minutes, a Navy helicopter was dispatched and Coast Guard cutter Sexton was diverted to the scene, along with a small boat crew and an HH-65 helicopter from Miami.