The girl was a 16-year-old runaway, police told the news station.
As Avila drove the group to the hotel, he said the women talked the teen through what was expected of her.
"They were describing what they were going to do when they get there – check for guns, get the money before you start touching up on the guy," he said. "I was like, 'There's no way. I can't just drive away. Of course not. It's not even an option.'"
A few minutes after dropping off the trio at the hotel, Avila called police. Officers descended on the scene and arrested Westley, Pettway and the suspected client, who was identified by The Sacramento Bee as Disney Vang, 20.
Speaking with KXTV, Elk Grove police Officer Chris Trim credited Avila with putting an end to a "very serious" crime.
Westley and Pettway are charged with unlawfully causing a minor to engage in a commercial sex act, pimping and threatening or encouraging a minor 16 or older to engage in prostitution. Jail records showed both women remained behind bars Thursday.
Vang was arrested on suspicion of having sex with a minor and released from jail Tuesday, the Bee reported.
"He could have said nothing -- went on his way, collected his fare," Trim told KXTL. "And then that 16-year-old victim could have been victimized again by who knows how many different people over the next couple of days, weeks, months."