Your son's attraction to this brat probably has a lot to do with hoping to get her on his side so she'll stop being mean to him.
I think the problem is unsupervised children. If that brat's mom was onsite all the time, she'd have fewer delusions about her kid's behaviour. And if you were there all the time, right beside your son, brat would have less access and more supervision than she has now.
We have a cultural story about children 'needing' other kids, and how it's beneficial for kids to spend periods of time roaming freely with bands of other children.
I disagree. Children make very poor adults, and that means they're bad at supervising themselves and other children. And, frankly, children need parents and adults who love them and who will keep them safe. Everything else is nice if it works and remains safe and healthy, but unnecessary.
Nowhere else in the world are children 'free' to spend the kind of time at liberty that the average American 6yo has. Strangely, all kinds of things are much less common in other parts of the world: bullying, gangs, the whole teen 'generation gap' problem, vandalism and surliness...
When I look around at the kids in my neighbourhood, who we've known for decades now, I see two groups: happy, well-adjusted young adults who engage readily with people of all ages, and angry, childish young people who are immature for their ages, who continue to wear clothing appropriate to children 10 years younger than themselves and who associate almost exclusively with people their own age with great disrespect of anyone much older and much younger. From this viewpoint, it's perfectly obvious which kids were either supervised most of the time by their parents or by the parents of their friends.
You could save that little girl, by mentoring her rather than trying to get her mom to parent her properly. Only if you feel you have time while doing your primary job of raising your son well, of course.