Fantine knew a lot about Heaven. She had been there for years, after all. When she first got there they told her she had to pick someone to 'watch over'. She was going to pick Cosette, but remembered that there is someone else, someone else maybe even MORE important.
Jean Valjean. He was important to her, she could not explain why but he was. Maybe it was that was the last person she saw on Earth, Cosette was very precious to Fantine but there was also another thing about Valjean.
She heard that when you 'watch over' someone, you can see their past. You can see their suffering. You can see their soul. Fantine knew what she would see in Cosette already, but Valjean? She knew nothing about him. She knew nothing of the secrets he hid, then.
But once she started to 'watch over' Jean Valjean, she saw the past, the suffering, the soul. And all she could feel was misery. She had had an awful life, at least for her final years, but Valjean suffered through all of it. And he never complained. He never wrote people off as being 'terrible at their jobs'. He never spat in their face. Fantine felt terrible.
But when Jean Valjean died, she could now speak to him. The first thing she did was embrace him, then say "You did it."
At this point, Valjean then realized what Fantine meant, and smiled. "She was a glorious child. More than a child, an angel."
Fantine continued to hug him, then say "I saw everything. From here. And I also saw your past. I know who you are. I know that you were a galley slave. Some would care, some would worry that their child had fallen into the wrong hands. But I don't. I would worry if she hadn't been with you."
Valjean smiled at this. "Let us watch Cosette and Marius. They are our children, as we are their parents." Fantine grinned and looked down at the young couple, still in tears because of Valjean's death. "Why do they care so much? Did I succeed?"
Fantine turned to Jean Valjean. "Obviously. Now, I have to tell you a few things. One: This is Heaven. Not the Heaven you might be thinking of, however. Everyone goes to Heaven. If you are done with Heaven, you can return to Earth. However, if you decide to be 'reborn', then you will forget everything about your past life. Two: You have to 'watch over' someone. I think I know who you will pick."
They both say at the same time: "Cosette."
"Okay then," Fantine concludes. "Let's begin."