What would you do (as a parent)?

It's a great opportunity for you to reinforce the good manners you have taught your daughter. Tell her after meeting her new friend, it makes you very proud to have a daughter who is considerate and polite.Tell her that you hope that through her friendship with this girl, you know she will help her to learn to become just as polite. You should treat the new friend politely as well. If she doesn't thank you for something, you should still say, "you're welcome" If she criticizes your car or something in your home, thank her for her opinion and point out that you are not her mother and she is not in her house, but a guest in yours. Positive reinforcement works better at this age. Forbidding a young teenager to do something gives them the excuse they are looking for to rebel.