Frankly, if that had been my daughter, she would've found herself punished (grounded) for her rude and disrespectful behavior. I don't behave that way when she brings her friends over - some of which I don't care for - she doesn't get to treat my friends like that. Plain and simple.
I like your "plain and simple" parenting style but have you ever consider to look at the whole thing from that child's point of view? May be the acting out is resulted from confussion and hurt because she is thinking where do I fit in between all this new mess of events and feelings. Why do I no longer have all my mother's time and attention. What the heck is happening?