Cory spent the night out with three friends and Andrew has insisted that the group did not have a negative influence on the Glee star.
"They weren't strangers, they weren't bad guys," said Andrew, speaking to PEOPLE. "They were long-time sober friends. Cory knew them maybe ten years.
"Essentially, he had spent time with [them] as sober people. They were not using with him [that night]. There was no double life up here."
After spending some time with his friends, Cory returned alone to his Vancouver hotel room and was found dead the following morning on 13 July, having overdosed on a lethal concoction of heroin and alcohol.
The Canadian-born had struggled with alcohol substance abuse problems from his teenage years, and was admitted into rehab when he was 19. He voluntarily checked himself into rehab last April and at the time was fully supported by his girlfriend and Glee co-star Lea Michele.
"I love and support Cory and will stand by him through this," said Lea. "I am grateful and proud he made this decision."
At the time of his tragic death, Lea thanked fans for their love and support and asked for privacy.
The brunette beauty and the rest of the Glee cast have now returned to work less than a month after Cory's death, and on Tuesday Lea posted a poignant photograph on Twitter showing her back on set.
"Feels so good to be home," the star wrote, alongside a photo of her in character as Rachel Berry, staring pensively into the distance as she sits alone at the end of a bed. Lea added that the show had chosen the "beautiful & perfect song" for her character's return.
The fifth season of Glee will premiere in the US on 26 September and will air a special memorial episode for Cory on 10 October.