His parents stated that they signed him up for an Intro to Backpacking Trek which is listed in the Buffalo Trail Scout Ranch High Adventure pamphlet.
On page 5, "Adult Leadership on Treks - all crews going on the trail must have adult leaders..."
Pages 6-9, detail the Introduction to Backpacking.
I realize that, but this wasn't a crew going out on the trail on a "Trek". The scouts in the "Intro to Backpacking" were still in the "base camp" portion of the week. They weren't out backpacking over-night, it was just a day-hike. My troop's camp-outs all follow the "Two Deep Leadership" rule, but if one of our patrols wishes to take an hour or two day-hike, they are allowed to as long as they clear it with the SM first. "Two Deep Leadership" has NEVER meant that all scouts will be accompanied by 2 adults at all times; it simply means that the over-all outing has at least 2 adults (one over 21).
So as far as I can tell, here are the relevant rules:
Two Deep Leadership: Rule met because 2 adults were supervising at base camp;
Buddy System: Rule met because the scout wasn't out on a hike by himself;
No One on One contact: Rule met because there was an additional scout along with the 15-year old and the 18-year old.
The only real issues here that I can see is "Did they get approval from the ranking adult before commencing the hike" and "Should the ranking adult have approved the hike given the weather conditions?"