I'm pretty sure the only "duty" in this scenario lies with the one who just sent his kid up on a frozen waterfall without really knowing what to do.
(We all make mistakes, of course.)
Sure, the dad is the person responsible for the kid's safety and is doing a bad job. If you saw the same guy in the parking lot telling his son to move a downed power line that was sparking so they could get their truck out of their parking space, you'd say something, right?
It's no different. It's not anyone's job to convince the guy, to stop the guy, or to educate the guy on safer behavior. I'm not proposing that anyone act like a white knight: only to say something.
"Hey wait, don't take him off belay, that could get him killed!" is all.