Ohhh I love the true love idea - revenge type thing! Always great drama with that. Perhaps the lady stole a bracelet/ring/whatever like Cohen has from his own witchy friend that gives her the ability to be out whenever she wants.
Oh yes! Love it. That way she can't fully trust anyone.
And I'm not sure either. I know why Cohen is going after her personally, but maybe she's mistreated someone from your witches past too?
Maybe we could throw something in about her wanting to put a stop to the same woman who turned Cohen for other reasons? That way they could almost be forced to work together...just a thought!