Jessalynn
You are not over reacting at all. I would have felt the same way, but unfortunatly I would not have been able to walk away and I would have made the kid go home. I also would have told the parents about it even if I was friends with them so they could possible get her to explain why she said it.
I have raised Mackenzie to be very independent. But at the same time she knows that when it is more then just her that you take turns doing what each person wants. Most of the time she will actually insit that they play what other people want because she does not want to make everyone play her things.
However if my child every said anything like that I would want to know about and then I would be talking with her about how we do not say things that are mean and hurtfull and that she needs to consider why the other person is crying and maybe correct the issue if she can or just go up and give her friend a

and ask what that friend would like to do. I would also make my child apoligize to the parent for saying No when asked to leave. If a parent asks you to leave you leave it is not an option.
Sorry just my thoughts. Let me know how it all goes within the next few days.