Teenagers' brains aren't even finished maturing. Probably because of that, none of was even thinking about watching out for his buddies. Kids see their friends as "on equal footing" . Some either didn't even notice the sweater, of at least "put two-and-two together" enough to think out what noticing it in passing actually meant. They just tend to be more careless, forgetful, and self-absorbed than more mature people. Some particularly mature teens may have noticed and said something, but that would be the less common, more mature, kid. I don't think their not thinking to remind their friend was a sign of bad parenting - just a sign that they're kids and kind of absent-minded, preoccupied, and/or thoughtless. Since they were kids, there's a good chance they're actually so concerned with what they do, and that things go OK with their boarding (etc.), that's all they "have room to think about".
If the same kids were at the same airport with the same forgotten sweater five years from now things probably would have gone differently. :)