Guyer reveals she booked her then-boyfriend into California's Betty Ford Clinic in the hope that Haim would clean himself up, but when he quit rehab after just eight days, she knew the fallen star was doomed.
She tells Life & Style magazine, "We almost knew it was going to happen... He had a devil inside him, and the devil was the drug."
The model and actress, who got engaged to Haim in 2000 after a whirlwind romance, adds, "He got violent and temperamental. He was doing a lot of drugs.
"I tried my best to help him. It’s very, very sad."
Haim died from an apparent drug overdose at his mother's Los Angeles home on 10 March (10).