Upon seeing the man knocked out, Jim rushed, even jumping steps on the way to help him. With the cover fire, he managed to carry him in a fireman's lift. When he reached the girls, he realized that they had to stop the zombies from advancing, one way or another. He put the man on the ground beside the girls, within their sights if he moved, and said "Don't let them get past you."

Then, Jim opened the nearest office. Luckily there were only two zombies inside, whom he easily dispatched. He then took desks, one by one and threw them towards the lower landing. When there were enough desks to cover the landing, he went down and arranged it so that there was a tall wall, a desk barricade that will halt the zombies' progress. If they needed to get down, they can jump. If they needed to get back up, they can climb, which the zombies probably could not do. The barricade would only probably fall to a huge, determined horde of zombies. 

Panting, he looked to the girls and showed them a thumb up, signaling that it was done.