I am 23, so not that long ago was i 14, and i remember that the fun and enjoyment was of doing something you shouldnt be. Like eating all the bsicuits etc and getting or trying to get away with it.
If you take that away, ie let her see him, let her drink etc, soon enough teh novelty wears off and its no longer exciting, and therefore she wont bother.
Certainly do talk to her about the intimate side of the relationship, and the possible implications and if you are really that worried about stds and pregnancy maybe even give her some"protection" to help her. It shows you trust her and therefore she will (hopefully) respect you.
My mum was laid back with boyfriends and alcohol etc, hence i first slept with someone at 16, who then became my husband and i first got drunk at 17, and i have only been drunk once since then, and that was last summer, aged 23.
My mum always told me that we had to make our oqwn decisions inb life, she could only guide us, and not live our lives for us, and be there to pick up the peices and dust us off when it all went wrong. She does, even now, my marriage recently broke down (due to dv) but i knew i could turn to my mum. She has helped me pick the peices up to start again.
I know a 14 year old being sexually active is wrong in so many ways, but the fact is that you cant really stop her. Before you know it, she'll be bunking off school to see him etc, its better really to say yes okay, of you go, and dont forget if you cant be good be careful!