I'd talk at her at such length and in such detail and with such vigour that she'd either agree never to see him again or, if I overdid it, to enter a convent.
My daughter's approaching 12 and I have two younger sons and I'm currently perfecting the art of talking them into submission.
'Let's go to the bedroom for a chat', they now realise, means that they're going to have to listen to me for a LONG time. Usually it works, thus far.