Dustin Moir was convicted in February 2010 of first-degree murder in connection with Acorn’s death.
The B.C. Court of Appeal ruled Wednesday that the trial judge erred in giving instructions to the jury about witness statements in Moir’s trial.
West and Moir were charged with first-degree murder in 2007 after hikers found Acorn’s remains in a shallow grave in a remote area just off the Coquihalla Highway.
Acorn had been reported missing from a group home in nearby Abbotsford in June 2005.
In October, West had told the court that Acorn's death was accidental. He had said that in June 2005, he saw his son and Acorn having an argument, when Acorn suddenly fell over. West told the court he took her pulse, and found that she was dead.