Why did West need an explanation? I have little sympathy for players who ask unnecessary questions and then call the cops to claim that the answers created unauthorized information for their opponents. West was not going to bid over 3NT, regardless of the answer to the question.
Law 16 says that the UI must "demonstrably" suggest the action taken, in order for the action to be barred. I don't think that "undiscussed" demonstrably suggests anything, at least in this auction. If South had said that 3S "shows a partial spade stopper," that would be different. Even "asks for a spade stop" (the meaning Alan Wilson imposes on the bid) suggests pulling more than "undiscussed," because then 3NT could be "just following orders" rather than a real suggestion of a final contract.
The whole situation is strange: South Alerted so that East-West would know that the bid was not discussed??? In ACBL-land the cue-bid is not alertable.
If we think that North bid 3S intending to play 3NT if South had a spade stop, then pulling to 4H is indeed suspicious. But that would be a silly thing to intend with that North hand: why would North expect South to have nine tricks after winning the spade lead (even though that appears to be true on this hand)?
It's not relevant to the allowability of 4H, but 3NT appears to be a better contract; it needs a diamond guess after a club lead, is cold after a spade or diamond, and is down only after an unlikely heart. In 4H, after two rounds of spades, North trumps and leads the HK, which holds. (Probably better to just lead small, but that's a hard play to find in real life.) North plays three rounds of clubs, West throwing spades. I guess North can get it right, but it needs perfect guessing - with hearts ideally placed at that.
I think North intended 3S to mean "I have extra values." South's 3NT simply shows a stop, no matter what North's bid is. I'd let North bid anything he wanted.