"He said, 'If you just go read this last message...I promise, you'll never, ever speak to me again, and I will stop contacting you,'" Piper recalled. "And I think at this time I was kind of delirious, you know, I was broken, and… [an] opportunity somebody would've given me to get this man out of my life, I just wanted him gone."
Still on the phone with Lynch, Piper headed out to pick up the message. He asked her to describe what she was wearing and to tell him when she walked out the front door of her building.
"He was like, 'OK, and, and what about your hair,' I said, 'It's up, I haven't showered,' you know... 'why are you asking me these strange questions now?'" she told "20/20."
At least three separate security cameras captured what happened next, but Piper remembers the fateful day all too well.
"This guy starts walking towards me, crosses the road, and he's coming right towards me, and his eye contact, he's so focused on my eyes," she said.
She was still on the phone with Lynch when she was attacked.
"Danny was on the phone listening to me screaming ...It's so painful and...So bad, and, and I knew straightaway that it was to do with Danny, he said he'd get me. He said he'd destroy me, and, and he'd done it," Piper said.
Piper's doctors began a series of reconstructive surgeries, including a revolutionary skin graft on her face. Beyond a forest of medical concerns, Piper also obsessively worried about the legal proceedings against her short-lived boyfriend and his henchman who threw the acid.
An ongoing police investigation revealed that Lynch had a long history of violence. He had even served a prison sentence for throwing boiling water into a man's face.
"I don't think anger even begins to cover the word of what I felt, really," Piper's father David Piper said.
"I try not to think about him, because I can't, I can't believe he is a human being," her mother Diane Piper added.
Six months after the attack, Piper testified at Lynch's trial for her disfigurement. She faced her attacker from just a few feet away and spoke out about the terrible role his attacks had taken on her mind and body.
"She was wearing a full plastic mask all over her face...We were feeding her through a bag and a pump because she couldn't swallow solid food. ...And the judge looked at her and said, 'Are you sure you're fit enough for this?'" Piper's father recalled. "And I looked at her and she looked pathetic. And I thought, 'After all you've been through, fighting for your life, now you gotta go through this.'"
A jury found Stefan Sylvestre, the man who threw the acid, guilty of causing bodily harm. He received a life sentence, with a minimum of 12 years in prison. Lynch received two life sentences, and will serve at least 16 years in jail.
Knowing that Lynch would be behind bars was an important milestone for Piper.
"She was very scared that he would come back and hurt her again," said Dr. Mohammad Jawad, who spearheaded Piper's treatment. "So when that thing happened, and, and they had given [the] sentence, I could see a huge change in Katie's world."