Fauchelevent is an old man whose life Jean Valjean saves, even though Fauchelevent has spent the majority of his time badmouthing Valjean. Rather than spend a long time showing us what Fauchelevent is like, the novel tells us:
Fauchelevent was an old man who all his life had been wholly self-centered but who now, at the end of his days, crippled, infirm and having no further interest in life, took pleasure in his sense of gratitude. (18.104.22.168)
When the man finally gets the opportunity to return Valjean's kindness and save his life, he's thrilled to get the chance. He manages to hide Valjean inside a convent until the threat posed by Inspector Javert has passed (at least temporarily). Now that's some solid payback.