Caderousse, stricken by poverty, salivates over the idea of having more than one-fifth of that diamond, and so he tells Edmond-disguised-as-Abbé-Busoni about the scheme that Dangler and Fernand cooked up so many years ago to get Edmond imprisoned.
When Caderousse spills the beans, Edmond-disguised-as-Abbé-Busoni realizes that Faria's suspicions and hypotheses about how Edmond wound up in prison were right all along.
Fortunately for him, Caderousse is genuinely wracked with guilt over Edmond's false imprisonment fourteen years ago. Edmond is touched by Caderousse's display of guilt and torment, and he (still disguised as Abbé Busoni) proclaims that Caderousse is the "dead" Edmond's only real friend.
He gives Caderousse the entire diamond, and, well, you can imagine how the old innkeeper feels about that.
OK, now to pour some more salt on Edmond's wounds: Edmond discovers that, after going to work for a Spanish merchant, Danglers then found his way to a Spanish bank and turned into Donald Trump. Apparently, he's somewhere in Paris, rolling in his millions of dollars at this very minute.
Our favorite brooding brooder, Fernand, has gone and married Mercédès. After he returned from being a soldier in Greece, he and the little lady got hitched. At that point, Edmond had only been in prison for a year and a half. They both believe that Edmond is dead, and they, too, make their happy home in the city of lights (Paris).
Sucks? You bet.
But, now let's add some vinegar to Edmond's wounds. Caderousse tells Edmond-disguised-as-Abbé-Busoni that his padre (Dantès Sr.) died of starvation – he was so sad that his son had been sent to prison that he didn't (or maybe couldn't) eat. Apparently, Mercédès and Morrel (remember him?) both tried to nurse the heartbroken old man, but he refused their help. During his last days, Morrel left a purse full of money for the old man, but the old man refused.
Edmond-disguised-as-Abbé-Busoni asks for the purse, and Caderousse, impassioned and heartbroken himself, proclaims the unfairness of life. Remember how Morrel was a successful merchant back in the day with ships and goods and a business? Well, every ship but one (the Pharaon) has sunk. Like Caderousse, he is completely broke – destitute even.
Caderousse reflects on how unfair it is that the good guys never win. Edmond-disguised-as-Abbé-Busoni responds that this is not always the way things go down. Wink, wink.