I could be just as childish, but in my sorority house, there was no going out with a guy who a sister went out with. I understand that he's your friend, but she's your sister, and would you want to have to deal with the situation if you were her? I'm not saying she's right...because I personally think she had her chance, but if it was me, I wouldn't bring him for the simple fact that I wouldn't want one of my sisters in a bad way. We all do stupid stuff...and we still love each other
That's part of sisterhood!