Christie Brinkley is opening up about her fourth marriage in her new memoir, Uptown Girl.

The 71-year-old model had previously kept the details of her split from Peter Cook private. However, she recounts the shocking moment in 2006 when she learned of Cook’s alleged affair with a much younger woman. While delivering a graduation speech at Southampton High School in Long Island, Brinkley was approached by an unfamiliar man who said, “Excuse me, I need to tell you that arrogant husband of yours has been having an affair with my teenage daughter.”

Brinkley recalled being so stunned she froze. She then found Cook’s face in the audience.

“Peter’s eyes were already fixed on me, shaking his head and mouthing ‘No,'” she recounted. “From his expression, I knew he was guilty. In that moment, I felt like I might faint onstage, in front of hundreds of people,” she remembered. “When I regained my composure, I asked the man for his card, but he said he didn’t have one and identified himself as a local police officer. He told me I could visit the station if I wanted the full story.”

“When your world crumbles and you suddenly find yourself the cliché of a middle-aged woman whose husband is involved with a much younger woman, what do you do?” Brinkley reflected.

She described her marriage to Peter Cook as “one of the most tormented experiences I’ve ever had.” The pair tied the knot in September 1996, just months after they began dating. They welcomed their daughter, Sailor Brinkley Cook, in 1998, who joined Brinkley’s two other children—Alexa Ray, from her marriage to Billy Joel, and Jack, from her marriage to Richard Taubman, which ended in 1995 shortly before her relationship with Cook began.

After discovering Cook’s alleged affair with a teenager—whose identity remains unverified—Brinkley ended the relationship. However, it took six years to finalize their divorce and custody arrangements.

In her memoir, Brinkley also opened up about her marriage to Billy Joel, which lasted from 1985 to 1994. She revealed that Joel’s drinking problem ultimately led to their separation, recounting how he would vanish for days during drinking binges, including one instance where he disappeared for two days after leaving Alexa’s fifth birthday party.