When my daughter was about 12 (1990), she had a friend sleep over and I was to drop them off to meet other friends at a theme park. Before I dropped them off, I had to do some grocery shopping. My daughter's friend made a snide remark to me about not dropping them off first. I said, "If you don't like it, we can just not go at all."
I found out later from my daughter that she let her friend "have it" over the way she talked to me. She told her never to talk to her mother that way. I was so proud of her. Instead of just siding with her friend and being a spoiled pre-adolescent brat, she stood up for me.
This same daughter left me a letter in my dresser after we dropped her off at college thanking me for the way we raised her and she understood why we grounded her at times when she didn't behave. If a mother ever had to come to me to talk about my any of my daughters' behavior, they would have been disciplined in addition to what that mother did.