I have no way of explaining why she went with them. We have no way of knowing what happened other than she was killed. Working with 20/20 hindsight, while it may make us feel much more sensible, and it may make us feel that we have prepared our kids so much better than she was prepared, simply is indulging in mental masturbation.
An 18 yr old girl was murdered by a coldblooded killer. That's the net result. Feeling better about ourselves somehow doesn't assuage the waste.
Since the group was only there for a week, I think it's safe to assume she knew him for less than a week.
Heck, I knew my own Hubby for only a few hours, before he called and we went out on a date in his car.. alone..I could have ended up dead, I guess.
sw