a fake friend - A fake friend is like a broken toy that only works when you'

A fake friend is like a broken toy that only works when you're around. They’re nice at first, but then they slap you in the face.

She said she'd be my friend forever, but she left me for the popular girls.

She told me she'd help me with my project, but she cheated on me.

She said she'd be there for me, but she was with her ex the whole time.