I've done several treks through Philmont, as have my father and brother. We've also spent a lot of time camping in and around that area. It can be a dangerous place, but I can see how something like this could happen. Kids and adults from all over the world go out there to hike. Once the guide leaves you're on your own. Judgement calls about where to walk, how close to an edge to get, where to sleep, etc. are different among people. This is a very unfortunate case, but I don't know that negligence is the proper term.
