Tell her "I know you say you won't have sex with him, but I just want you to think about what would happen if you did have sex, and then he broke up with you the next day, or told all his friends all about it. Or what if you got pregnant? Imagine how upset everyone would be. Also, be careful about anything that could lead to sex, sometimes you think you are just having fun and then you may feel unable to say No, and things may go a lot further than you planned, and you won't really realise until after, and it may make you feel tricked. Just think about these things, and make sure when you go out on dates that you actually go out to a public place, rather than just sitting in his car or whatever, just to take that pressure off.'
These are the kinds of things I wish I'd been told when I was a kid... Hope that helps.. Don't be afraid to just bombard her with advice, as long as you say it in a sisterly way, not a judgmental/patronising way, it may sink in just a tiny bit more.
I have a 16yo cousin and she now asks me for advice before every date etc, she finally listens to me, so I think it may help? Good Luck!!