I know a woman who did this. It was a while ago, and the kids are teenagers now.
The mum was lovely, a really nice person. She was friendly, kind, generous and good company. I'm not sure if it was called UP then, but she certainly didn't set any boundaries. Didn't even nicely remind her child to say hello, please or thank you. Didn't tell the child off if she snatched something, hurt someone, called someone names, was rude to her etc. The child was an absolute horror. The teachers had huge problems getting her to do what she was told, none of the children liked her or wanted to play with her. Other parents would invite her over to play and she would be extremely rude, and behave badly. I know this as I was one of these parents! Now perhaps the child would have been difficult anyway, but all the other parents were amazed when this child was never told off.
They moved her to a different school, because the mum thought she was being bullied. I don't think she was, I think she just alienated all the other kids.
Since then, the parents have split up, and I don't know what happened, but the child now lives with the father, and will not speak to her mother or have any contact. I have bumped into the mother a few times, and she is heartbroken. As I say, she was a lovely mum, just needed to be more assertive in my opinion.