The passenger may have brought his own. They asked him twice not to eat them.
I have been on flights where they said “we will not be serving any nuts on this flight because of a sever peanut allergy, and we ask you not to open them if you brought your own”
That’s fine.
But would it not have been much safer to simply ask the parents not to board the flight?
If so sensitive, putting away what was once already opened would still be dangerous.
It’s the parent that initiated the risk here.
And that might even be what the guy with the peanuts was thinking - “Really!?! Then how did you make it through the airport?”
If they had shown up with requisite protection, which would be needed if the sensitivity claim is true, then maybe the guy would’ve realized it was indeed serious and complied.