"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," Montag, 23, Tweeted Tuesday about Dr. Frank Ryan, who performed her 10 procedures last year.
"He was the most brilliant talented surgeon who will ever exist," she adds. "My thoughts and prayers go out to his mother, family, friends, and anyone who was ever blessed enough to meet him. He is in a better place."
Ryan died Monday after his Jeep careened off the Pacific Coast Highway, according to TMZ.com.
While an autopsy is underway, the former Hills star said her doctor lived an alcohol and drug-free life. "FYI Dr. Ryan never drank, never smoked," she Tweeted. "He's never done drugs this was something everyone knows about him. He was always on call 24/7."
"We are all in shock," Ryan's rep tells PEOPLE. "He was such a hardworking philanthropic man, a loving son, and responsible for changing many people's lives for the better. He will be missed by many."
Reporting by JENNIFER GARCIA