The video, which was posted to a closed group on Facebook for Corona residents, shows the man holding onto the truck with just one hand while busting a move.
The man eventually dives head-first back into the truck through the open passenger-side window.
"My only thought was how stupid could you be," Frerichs told KABC-TV. "He could have easily fallen, I could have ran him over, it could have caused more accidents."
CHP investigators said the man could face a seat belt infraction and both he and the driver could face an unlawful riding infraction.
Frerichs said she posted the video in the hopes of getting a message to the man.
"I didn't post it hoping they would get in trouble. I was hoping that someone who knows them would say, 'you're pretty stupid,'" she told KNTV.