Ryan, whose clients included dozens of celebrities, but was best known for a marathon 10-procedure makeover on Montag, was sending a Twitter message about his border collie just before he wrecked his Jeep on the Pacific Coast Highway near Malibu, Charmaine Blake told People magazine.
"He lived up in Malibu on a tiny street and he was texting while driving and he accidentally went over the cliff," Blake said.
The California Highway Patrol confirmed Ryan was texting before the crash.
"It is one of the elements that we are investigating," CHP Officer Steven Reid says.
Blake, 29, who said she lived with Ryan from 2003 to 2006, hopes people learn from Ryan's death and don't text while driving.
"It was the only serious relationship I ever had and he was the love of my life,"she said.
Ryan, 50, whose other clients included Gene Simmons, Vince Neil, Adrianne Curry and Shauna Sand, was killed Monday afternoon when his Jeep went off the side of the road. He suffered major head injuries and died at the scene. Just before the accident, Ryan had been hiking near Malibu with his Border Collie, Jill, and shared a photo on Twitter.
"After 25 years of driving by, I finally hiked to the top of the giant sand dune on the pch west of malibu," he wrote. "Much harder than it looks! Whew!"
It was not clear if that tweet was the one Ryan was sending when he crashed.
Montag, whose dramatic transformation drew condemnation as an example of the wrecthed excess of Hollywood plastic surgery, tweeted after learning of Ryan's death.
"I am devastated to hear the news of Dr. Frank Ryan's death," the reality TV star wrote. "He was the most amazing person I have ever known. He was an angel and changed my life and the lives of everyone he met. He was the most brilliant talented surgeon who will ever exist. Dr. Frank Ryan changed the world."