The 32-year-old's cousin, Myriane Rodriguez, said she just dropped the children back at the apartment when she heard Hunter and Lewis arguing.
She then desperately tried to get help, but it was too late.
"I knocked on everybody's door, nobody answered their doors," Rodriguez told KHOU.
"I heard gunshots and it was my cousin in my passenger seat."
"Why, why would he do this. She loved her three beautiful kids."
Pictures from the scene show the white SUV with what looks like six bullet holes in the window.
Shoes that were being worn by the children when they left were also seen in the parking lot.
Police said the gunman, alleged to be Lewis, then fled the scene. Investigators said he drove to a motel a few miles away, before calling police to surrender.
Lewis is charged with murder, and he appeared in probable cause court on Monday, Click2Houston reports.
During his brief hearing, the judge heard Lewis admitted to arguing with Hunter and grabbing the gun.
The court was also told he said he did not remember pulling the trigger, but said: "I must have shot her."
Bond was not set for Lewis.