A beautiful brunette more than 30 years his junior, she had been his girlfriend for 2½ years.
The 23-year-old had been out for dinner with Nemtsov on Friday night. She had suggested getting a taxi, but Nemtsov didn't want to, preferring to walk, Duritskaya's mother said.
As the pair crossed a bridge in central Moscow, the 55-year-old Kremlin critic was gunned down by unknown attackers.
His girlfriend was holding his hand and heard what sounded like a clapping sound, her mother, Inna Duritskaya, told CNN. Nemtsov went limp and fell to the ground, fatally shot in the back.
Duritskaya is the only known witness to the shooting, which occurred 100 meters (330 feet) from the Kremlin but appears not to have been captured on security cameras.
Inna Duritskaya said her daughter called her right after she called police to raise the alarm.
"She was crying and she was saying Boris had been killed and he's lying next to me," Inna Duritskaya said. "She was in such shock she couldn't say anything else. Just, 'Mum, Boris is killed, and he's lying next to me.' "
She told the interviewer that she did not see who killed Nemtsov, but that when she turned around, she saw a light-colored car driving away.
In an interview with CNN, Ilya Yashin, a political ally and friend of Nemtsov's, said that Duritskaya had telephoned him immediately after the shooting and said there were several men in the car containing the shooter.
What's happened since?
Over the weekend, Russian authorities questioned Duritskaya about what she saw.
While in Moscow, the model was staying at the apartment of an Nemtsov aide under constant police guard.
But Moscow's Investigative Committee insisted that she was not detained and could have left at any time.
"She said she wanted to give the testimony and took part in investigative activities understanding the importance of it for the investigation," a statement released Tuesday said.
"We received no complaints from Anna Duritskaya or her lawyer during her stay in Moscow. Only today, after investigative activities are over she left Russia."
The model left the Russian capital Monday night bound for Kiev, Ukraine, said Yevhen Perebyinis, a spokesman for Ukraine's Foreign Ministry.
Inna Duritskaya told CNN that while being questioned, her daughter faced constant psychological pressure from Moscow authorities and that she fears she will be implicated in his death.
In a Skype interview with Dozhd, Anna Duritskaya indicated her continued presence in Moscow after the shooting was not from choice -- and seemed to appeal to be allowed to leave.
"Now I have a status of a witness, I gave all evidence possible, I don't understand why I'm still on the territory of Russia. I want to go to my mother who is ill, who is in very difficult psychological condition," she said.
"During three days, they have been taking me together with security in police cars to the Investigation Committee and conduct the investigation. They are not explaining when they are going to release me and the reason they are keeping me here."
Are we likely to learn more from Duritskaya?
According to the Investigative Committee, Duritskaya signed a witness protocol and confidentiality agreement, which is standard procedure in Russia.
It states that she will not share information she gave to investigators or any preliminary findings of the investigation, the committee said.
"Leaving Russia, Duritskaya expressed a wish to continue to cooperate with the investigation and to participate in any ongoing investigation if needed," it added.
What was her relationship with Nemtsov like?
Inna Duritskaya said her daughter knew the risks of getting involved with the prominent opposition leader.
"Of course when I heard she was dating him, it was a bit of a shock," she said.
"Then when I met him in person, I liked him a lot as a person. He was straightforward, he treated my daughter very well, she was happy. And I couldn't influence it anyway; my daughter did what she wanted to. Of course, I was worried and scared, but I couldn't change it anyway."
Duritskaya's reported return to Kiev means she was unable to attend her partner's memorial service and funeral Tuesday.