Samantha Taylor broke down and cried as she talked about the paralympian's obsession with guns and the reasons their relationship failed.
"He cheated on me with Reeva Steenkamp," Taylor said.
Pistorius' ex-girlfriend was called to testify about an incident in which he allegedly fired a bullet through an open sunroof of a car.
In testimony earlier this week, a friend said Pistorius accidentally fired off a gun in a restaurant about a month before the killing, then convinced another member of the dinner party to take the blame.
Pistorius' murder trial will resume on Monday. With less than 10 percent of the witnesses on the list already having been called, it is expected the trial will last several weeks.
If Pistorius is convicted of murder, he could face 25 years in prison before he is eligible for parole.