How can I deal with this psycho girl?

Next time you see her at the next gathering if she starts beef tell her very loud to cut her crap you are sick and tired already. Say you said I could go out with him and now your acting like a dam psycho. Ask her what the hell her problem is and he was not even worth it. She is not your real friend and I would not even stress her. If she was hurt about it after she said to date him then she should of said something maybe like I didnt realize it was going to bother me but it does. You would of ended it just like you did cause you said it wasnt worth it. She didnt even give you that chance. Good Luck