It sounds like you were responsible for her daughter, correct? She has the right to be upset (and I am not saying with you but in general) if her expectation was that there would be more supervision of her child while she was in your care. She was upset. She was worried. She's that child's parent. So she doesn't have the same level of comfort as you did in that situation. That's her prerogative as a parent. So what? Are you worried about how you look now to her? You shouldn't. You are the kind of parent you are and she is the kind of parent she is. If she allows her child to come over again she might just give her more explicit instructions beforehand about how she wants her to conduct herself while there. Check in with mom maybe before making a decision. Who knows. It's a learning a experience for them it seems. Does not mean you should change your parenting style or her hers though.
Sorry but when a 16 yr old is with my "child" they are responsible for themselves and dealing with their own parents!!! I don't babysit 16 yr olds. 16 yr olds are not in my care! They should know what they can and can't do and if they do it or not is between them and their parent. If the parent doesn't like it she has a problem with her child not me. I never in my life asked my kids friends if they had permission to go somewhere from my house. You just don't do that with 16 yr olds. In fact they usually only say hey we are leaving now, going for food, get a movie, etc. I certainly didn't ask each one if they had permission. I have to say this again you don't babysit 16 yr olds, they are not in your care like it is a play date for 5 yr olds.
) and I think a large part of the reason is that they have been allowed to truly test and assess their limits. They are the kids who happily talk in front of the class, who are very outgoing and friendly with kids and adults alike, and are always referred to by teachers as hardworking and natural leaders. Although let me tell you, having a house full of "natural leaders" can be a bit of a pain in the butt when they all want to lead at the same time!