Las Vegas police Lt. Ray Steiber says 33-year-old Jesus Manuel Magdaleno Jr. and his 31-year-old friend Felix Brandon Cruz were in Las Vegas for Magdaleno's bachelor party.
They were trying to stop the theft when they were thrown out of the bed of the pickup when it crashed about 11 a.m. Sunday.
Friends and family members are now with Cruz at University Medical Center.
He's hospitalized with a critical head injury.
Police say 29-year-old James Robert Montgomery of Tustin, Calif., is jailed on felony murder, kidnapping, auto theft and other charges.