But it is also essential that Cameron "steadies the ship" , as promised, before stepping down in October.
With astute political management Britain, which is the fifth-biggest economy in the world, could quickly mount a rapid recovery from its new position as the world's leading political tourism disaster hot spot.
Here's how Key could help.
Instead of hand-wringing over the British people's decision to give two fingers to the decidedly sclerotic European Union (which Key did not really factor into his own political calculations), our prime minister should move quickly to acknowledge the courageous move by the British people to reassert their sovereignty. "The people have spoken" - yada; yada; yada.
To do this Key should immediately seek for New Zealand to negotiate a free trade deal with Britain.
Give Cameron a call and put it to him.
Both countries are well-equipped to forge a modern agreement which is shorn of all the political trade-offs which bedevilled the Trans-Pacific Partnership. Such a deal would also place New Zealand in a more powerful position in its own negotiations with the EU.
It would be ridiculous to mimic President Barack Obama's pre-Brexit position that said Washington would have no interest in negotiating a trade deal with a Britain that had left the EU.
A week ago, Key revealed that he was an ardent student of the Financial Times poll of polls.
He said that had he been asked three months ago, he would have put a "pretty healthy wager" that Britain would stay in the European Union.
But Key was amazed by the polls. Some showed Britain was likely to leave the EU. But his view was that people were "innately conservative" when they got into the polling booths. That had happened with the Scottish referendum and at the last UK election where the Tories got back against the polls.
On June 24, the FT's poll of polls on the EU referendum showed 48 per cent in favour of remaining; 46 per cent for exit.
What Key also highlighted was the "BeLEAVE in Britain" campaign which the Sun ran. The paper had successfully backed either the incumbent prime minister or the aspirant since 1971. But he still thought it was a big call against people's conservative instincts.
Key's musing aloud is instructive.
Ideologically, Key is more at one with Cameron and Australia's Malcolm Turnbull than, say, Obama.
He chairs the International Democrat Union; an organisation that brings together more than 50 centre-right political parties.
Where Key differs from his ideological soul-mates is the way in which he deliberately feints left to occupy the middle ground with voters.
The issue all three leaders face is that the middle-ground has shifted again.
There are big issues that worry people: The future of work in an era of huge technological change; too much immigration and the concern that this will lead to enormous social costs (in New Zealand is has resulted in the dream of home ownership being placed beyond many); and worries about personal safety.
We tend not to debate these issues in New Zealand. Our PC culture prevents that. That's not so in Britain where the decision to "close" the borders (which Brexit boils down to) has a certain logical attraction.
Turnbull's fate will be determined at next Saturday's general election,
Yesterday, Turnbull tried to capitalise on Brexit by saying: "There are always going to be challenges and headwinds in the global economy and that is why you need to have strong and stable government here in Australia."
It's likely that the Australian people will take Turnbull's soothing words with a grain of salt. If the Liberals are swept to power they will still face the mounting distrust within Western economies which Donald Trump so brilliantly leverages in the US.
The British economy has been riding high. It makes up 4 per cent of global GDP.
The Centre for Economics Business and Research has predicted that Britain will leapfrog Germany and Japan in the next two decades to become the world's fourth-largest economy boosted by its leading position in global software and IT sectors.
But that was before the disruptive effect of Brexit which could lead (as the centre predicts) to a "more insular and less diverse culture which in turn would generate slower growth."
New Zealand is in a position to assist Britain to stay outwardly focused.
This is the time for overtures not recriminations. Key take note.
