I'm on Xbox 360, so not sure if any of this will work - but worth a shot.
1. You can try holding down "A" with your cursor on her, which will bring up a command of "Wait there". Then after telling her to wait somewhere a few steps away try talking to her and see if you can tell her to follow you.
2. Other then that you could try hiring another follower, if they say you already have someone following you I would try waiting till it says your follower has tired of waiting and has returned home.
It all depends on the dialogue choices you have, if there's a choice of saying "wait here" then try option 1, just experiment around - try anything you can think of.