When Cameron returned from a visit to the Jaffna territory in the north, he described the "harrowing" experiences of the people he had met there.
New Zealand's Foreign Minister, Murray McCully, came back talking about new roads and infrastructure, newly elected Tamil provincial leaders and briefly noted there were still a lot of military there.
He then went straight from his visit to the north to warmly shaking the hand and gripping the shoulder of Rajapaksa's son, Namal Rajapaksa, who had decided to set up a Sri Lankan All Black fan club.
Namal is the captain of the Sri Lankan rugby team, and a member of Parliament. He is also seen as the successor to his father in politics. McCully was working it because should that happen, his apparent affection for the All Blacks could work in New Zealand's favour in the long term on the trade front.
Many had expected Key to throw his shoulder behind Cameron.
But while Cameron was alternating between Twitter and megaphone diplomacy, Key was cooing over Sri Lanka's elephants for Auckland Zoo.
Rather than issue ultimatums, Key was offering coaches and cows: the warm fuzzy diplomatic weapons of agriculture and sports that New Zealand uses to boost links with good trading partners. There was a new dairy partnership under which New Zealand would provide technology and expertise to Sri Lanka's farmers.
McCully's support of the Sri Lankan All Black fan base brought with it a promise to help improve Sri Lanka's chances to qualify for the Rugby World Cup in 2019 with coaching and player help.
Various motives have been attributed to Key's softly, softly approach in Sri Lanka. The most common was that New Zealand was more interested in getting Sri Lanka's vote for the Security Council and preventing any damage to trade relations.
There probably was an element of the second at least - Sri Lanka and Fonterra have only recently reached an uneasy truce following the contaminated milk problems of last year.
But it was one of the toughest diplomatic tests Key has faced, and his handling showed he is as much a pragmatist in international diplomacy as with economic matters back home.
Key pointed out that New Zealand already knew genuine reconciliation took time, and often generational change. He pointed out it was also hard to force it on a country which did not necessarily believe it needed it, and where many were simply thankful for the respite from the war, the suicide bombings and killings that afflicted the nation for nearly 30 years.
Having done his cost-benefit-ratio calculations and perhaps drawn on his experiences from dealing with Fiji, Key may have simply decided that trying to back someone into a corner could be counter-productive.
This may indeed by true - Rajapaksa only got more defiant in a bid to prove he would not be pushed around as Cameron upped his rhetoric.
Now the Commonwealth Heads of Government Meeting is over, and with it the end of the focus on Sri Lanka. David Cameron has moved on to tweeting about cyber safety and the bet he lost to John Key over the All Blacks-England rugby game.
As for that three-way handshake, Key did show he could drop the diplomatic niceties if pushed too far. After two years of mockery, he showed journalists a re-enactment of the handshake, claiming that he and Richie McCaw had been ready to engage when Bernard Lapasset came over the top. It would have been rude to mount a "J'accuse" at the time. But at least we know now Key can do so when pushed.
