Westerfield, 50, is accused of abducting and killing 7-year-old Danielle van Dam and could face the death penalty if convicted. The child's mother reported her missing the morning of Feb. 2.
Brenda van Dam told prosecutor Jeff Dusek she disclosed to officers everything about her "open marriage" once they made it clear that it was important.
The mother said she didn't discuss her decision not to disclose information with her husband.
"I would have told (the police) anything they needed to get Danielle back," the mother said.
Brenda van Dam said nothing else was taken the night Danielle was abducted.
"I wish they'd taken everything else but her," the mother said tearfully.
Earlier in the day, Brenda van Dam testified there were never any sex parties at her Sabre Springs home.
She later said that during a Halloween party last October, she and her husband had sex with a friend and her husband in the van Dam home.
Westerfield, a self-employed engineer, is accused of kidnapping the second- grader, killing her and dumping the body near rural Dehesa Road, where it was found more than three weeks after she disappeared.
Brenda van Dam broke down a minute into her testimony, when asked to name her children and give their ages, then later while testifying about a father-daughter dance Danielle was scheduled to attend with her father a week after she vanished.
Dusek asked the mother about the first of two "girls nights out" at Dad's Cafe in Poway with two girlfriends. Dusek asked if any men were invited to the Jan. 25 get-together at Dad's Cafe. Van dam said no.
"Was there a sex party at your house when you got home?" Dusek asked, hoping to pre-empt an anticipated defense attack on the mother during cross-examination.
"There's never been a sex party at my house," she responded matter-of-factly.
She also described previous contacts with the defendant -- she had said he bought her a drink at Dad's -- including a visit to his house with Danielle and her younger brother to sell Girl Scout cookies.
While the children were playing in his back yard, she said the twice-divorced Westerfield told her he was interested in her friend, Barbara Easton, who had caught his eye at Dad's.
On the second outing to Dad's, Easton walked right up to Westerfield and began talking to him, van Dam told the court.
Brenda denied dancing with the defendant on either occasion.
Earlier in the day, Medical Examiner Brian Blackbourne was asked about the cause and time of Danielle's death.
The girl's body was so decomposed when found by a volunteer searcher that it was impossible to establish a cause of death, Blackbourne had said Wednesday.
Thursday, he estimated her time of death at anywhere from 10 days to six weeks before the body was found Feb. 27.
Blackbourne also said it was possible that air temperature could be a variable in judging when she died. That led defense attorney Steven Feldman to suggest that especially hot days in late February could have accelerated the decomposition process.
In his opening statement Tuesday, Feldman told the jury that Westerfield could not possibly have dumped Danielle's body after Feb. 6, because he was under constant scrutiny by the police and the media.
"Is it your professional opinion (Danielle) could have been alive Feb. 6?" Feldman asked.
"Based on my observations? Yes," Blackbourne said.
"Feb. 7?"
"Yes."
"Feb. 17?" Feldman pressed.
"Possibly, yes," Blackbourne answered.
"Depending on the weather conditions, Feb. 22nd?" Feldman asked.
"That's pushing it for what I can accept," Blackbourne said.
He also testified he checked Danielle's body for signs of sexual assault but found none, primarily because of tissue damage from decomposition and animal activity.
Wednesday, the child's father admitted he lied to police about use of marijuana in the garage of his house the night of Feb. 1 -- hours before his daughter turned up missing.
He also told the court that on prior occasions he engaged in sex with both of Brenda's female friends.
A bartender at Dad's Cafe in Poway characterized Westerfield as a "people watcher," but termed a police report on the behavior of Brenda van Dam and two friends inaccurate.
Sean Brown said he was at work as a bar manager on the two nights that have come up in testimony in the case: Jan. 25 and Feb. 1.
It was Feb. 2 that the Sabre Springs 7-year-old was discovered missing.
Westerfield would talk to a regular customer, "hang out" and watch people as they drank and danced, Brown said. The bartender said he never saw the defendant dance or play pool.
Brown said Westerfield would drink 16-ounce rum-and-cokes.
"I believe David had a few drinks," Brown said of Feb. 1. "He was feeling good. He'd taken the edge off. I don't believe he was drunk."
He said the same about Brenda van Dam.
"I don't think she was drunk," Brown testified. "She was in good spirits but was in control of herself. She wasn't slurring."
Under cross-examination by co-defense counsel Robert Boyce, Brown said a police report containing his statements on the behavior of Brenda and her two friends was inaccurate.
Brown testified he warned prosecutor Jeff Dusek about the discrepancy during the lunch hour -- hours before he testified.
The statement from police indicated the women were "flirtatious toward males" and "partying hard."
Brown testified that the women were having fun as girlfriends do. He said he never saw them act inappropriately.
Brown was dismissed as a witness as the court day ended.