One of my friends met up with a man off the internet..?

I'm a guy, have a lot of female friends, and get very protective over them. One of my friends who's seventeen, got on the train on her own to HULL (we live in Cambridge) to meet up with a 23 year old MAN she'd met over the internet. Obviously I'm distressed but relieved she got back safely - but the worse part is, she never told me she was going. I wouldn't have found out had I not seen the photos she posted on MySpace of the two of them kissing. I feel worthless as a friend and I really don't trust the guy. What do I do?? I just want to be a good friend.