Many people ask if the friendship between Duryodana and Karna was genuine. Well, it started off for selfish reasons when Duryodana wanted a top class archer on his side for war to defeat his nemesis Arjuna. And Karna shook hands with Duryodana because he promised him kingship and would no longer be addressed as the son of a charioteer. What started off as a symbiotic relationship evolved into trust and loyalty and they became good friends over time.
This story illustrates their friendship. After Karna's coronation as King, he spent the evenings with Duryodana. And for leisure, they would play a game of dice. One such evening after dusk, the both of them were playing dice. At one point Duryodhana had to leave for a moment. Duryodhana's wife Bhanumati, who entered the hall found Karna waiting and took her husband's seat and played on his behalf. Both of them were in a jolly mood and enjoying the game. Bhanumati was losing and a small casual quarrel began about who would roll the dice next. Playfully, Karna tried to snatch the dice from Bhanumati's hand, but by mistake grabs her drape pulling her forward and the pearls on the drape break loose. Thus, Bhanumati's dress gets misplaced and her pearls scatter everywhere on the floor. At that moment, Duryodhana enters and finds his wife with his friend in a compromising position, clothes entangled and pearls everywhere on the floor. Bhanumati is embarrassed and scared and Karna finds himself in a fix. Duryodhana then asks Karna what they were quarreling for. Upon knowing the reason he makes a joke and laughs out loud. They continue the game.
Later that night, Bhanumati asks Duryodhana why he did not doubt her in spite of what he saw. To which Duryodhana replied, "In a relation there is no space for doubt, for when doubt arises, there will be no relation. Karna is my loyal friend and I trust him because I have faith that he will never break my trust".
Such was their friendship. When Karna died in the battlefield, Duryodana lost the will to fight. The only reason he continued was to get done with it. He wanted to die in the same battlefield where he lost his friend.