It's clear that South had shown his minor suit lengths then they would have defeated the contract.
It is very possible that the lead of the ace asked for attitude and not count - a popular method - in which case I'd judge North at fault.
On the other hand, if count was expected then I expect South was trying to prevent a desperate diamond switch, or trying to get his partner to underlead the
♣K so that he could push a diamond through.
However South should realise that partner is pretty strong (given his hand and the fact opponents are not in game) and that partner should not switch to diamonds unless it is safe to do so, so showing length correctly is probably best.
I think that if the lead demanded attitude, there is insufficient reason for South to play differently. Sabine Auken discusses this problem in her recent book (
I Love This Game) - essentially the opening leader's partner should not sit and think for ages due to the unauthorised information that it gives: this might be an example, if Ace demands attitude, then a very slow
♣7 would show
♣Q7x. Auken's method is to signal what you are supposed to and let the leader work out the defence unless it is completely 100% clear.
Paul
-- -- P P
1H 2N 3H P
P P