Some other angles you might consider:
(1) Are you sure what she’s written is what is really going on in her life? I checked out the opendiary.com website and find it odd that teens would post true stories about their lives in such a format (but hey, I’m probably out of it).
Could The Cat have made exaggerated diary entries and left the bookmark and the entries there to test you? (Heh! This will really get mom’s goat!!)
(2) How likely is Mr. DDG to find the same bookmark and read the diary entries himself? If so, The Cat needs to be aware that she’s left a trail to her diary and will apprecite your telling her about the trail.
You might consider asking her about the bookmark next time you are on the computer. “Honey, do you know what this is?” Of course, you’ll have to be prepared for some quizzing (“Did you open it? Did you read it?”), but her reaction might be enough to tell if she’s really been doing the things that are written in the diary.
If you have strong reason to believe that she’s really doing what is written in the diary, then I agree with the other posters who suggest you have the sex talk with her and be clear about how much STD’s and pregnancy’s can wreck peoples’ lives. Also, don’t fret the atheism stuff too much. Kids need some way to rebel against their folks, if not this, it could be something worse.
Finally, I recall being scared that my folks would find out that I wasn’t the perfect kid they thought I was. I was scared that I’d get in trouble with them. I was scared that my dad would think up truly horrible punishments (as in groundings and extra chores, not physical harm, btw). I’d wager that’s what The Cat means when she writes that she fears dad.
Good luck.