The duo were cycling across the California campus after midnight on Jan. 18, 2015, when they say they came across Turner "thrusting" atop a partially clothed woman.
"It seemed OK at first," Arndt told NBC News. "Then when we looked closer, Peter started noticing — and I later — that she wasn't moving."
The students approached Turner, who Arndt said stood up and left the young woman motionless on the ground.
"We saw she wasn't moving still. So we called him out on it. And the guy ran away, my friend Peter chased after him," Arndt told CBS.
He said Jonsson tackled Turner to the ground, while Arndt tended to the young woman.
"She was unconscious. The entire time. I checked her and she didn't move at all," Arndt said. Turner's attorney had argued in court the woman was awake during the encounter and consented.
According to police reports obtained by Huffington Post, witnessing the assault was an emotional ordeal for Jonsson, who cried multiple times while giving his testimony to police.