My friend is being a total *****?

Sorry if this is long but please bear with me! So basically I was really good friends with this girl (Iet's call her Millie) but now all of a sudden she's gone off with my worst enemy (let's call her Voldemort lol xD). Don't just say we can all be friends because I can never forgive Voldemort for what she did to me. Me and my other friend (let's call her Ellie) were getting really annoyed because Millie always said she hated Voldemort like me and Ellie. So we confronted her and asked her if she liked her because she always hung out with her. She said she does like her and we asked why she pretended to dislike her. Millie said, "Because...oh never mind!" and ran over to Voldemort. Now she sits with her in every lesson and she ignores me unless it's a Wednesday afternoon when we have a club together. She has no other friends in our club so she hangs around me and pretends to be nice because she's shy and doesn't really know who else to hang out with. What shall I do? Shall I ditch her? Do you think she was just using me and Ellie until she made other friends? Thanks