Just as Cosette’s mother gives her up hoping she’ll have a better life, so does Marius’ father. And both miss the chance to reunite before their parent dies. But Valjean never fills in Cosette on the details, while Marius is able to meet his father’s friends.
Marius doesn’t do anything by halves. Royalist until he learns his grandfather has been lying about his father his entire life, then revolutionary. Head over heels in love with Cosette, then suicidal. Rejecting his father in law the fugitive, then seeking him in gratitude. No wonder he’s so conflicted over his debt to Thénardier when he learns how awful he is. Marius is obligated to help him and to stop him, and absolutely must do both.