How can I ditch my best friends friend!?

My best friend, Connie, has another best friend, Conschi. Well we all used to be best friends but frankly I don't think Conschi is a good friend at all. I don't want to be her friend but I don't want to be a ***** either. On friday Conschi and I are going to spend the night at Connies house...and I want to gradually part from Conschi. She's hurting me all too much and I just want her gone. (not like dead but just a friend who says hi in the hallways) How do I do this when she's Connies friend too?