What we do know (based on the articles) is what the mother has told a reporter. I don't for one minute believe it's the whole truth and there is evidence there that confirms that (life ban? or 2 year ban?). I discount the other pax telling the miscreant not to eat his nuts as the mother could not possibly have heard that conversation, or she wouldn't have waited 20 minutes to do something about it and other than hearing it, is just hearsay. I also doubt the crew specifically told the guy not to open the nuts. Did they go right through the whole cabin interrogating each person individually, to ascertain who did and who did not have a concealed pack of nuts? For that matter, are we all convinced the lady two rows behind her wasn't happily snacking on some nuts?
I also think, perhaps incorrectly, that a duty of care is only owed to others with regard to reasonably anticipatable or foreseeable hazards. If the nut man didn't understand the instruction than I'd be assuming this particular hazard and risk was not reasonably foreseeable. In fact, I tend to think the nut man may have a genuine action against the airline.
I do say though, I'm a fence sitter on this issue. I am glad the little girl is OK (or is she)? Someone mentioned earlier there may be a risk of a brain injury for the amount of time she was oxygen starved. I've got no idea but I hope not.