While Frederick is away, Deslauriers begins dwelling on things. You know, how Frederick has it so good—the money, the looks, the connections. Why does he always get the short end of the stick?
He decides to try to win the post with the Dambreuses that Frederick was meant to fill. He also thinks about wooing Madame Arnoux. He's tired of hearing how lovely she is, so he decides to win her over for himself.
Now things are getting exciting.
Deslauriers justifies all of this by telling himself: "After all, even though he should get angry, so much the worse! He has behaved too badly to me to call for any anxiety about him on my part! I have no assurance that she is his mistress! He has denied it. So then I am free to act as I please!" (2.12.15)
He shows up at Madame Arnoux's house, claiming that he's there to discuss the loan from Frederick. She's surprised that he's acting so disloyal to his friend. He tries to butter up Madame Arnoux and then makes an incredibly clumsy pass at her and basically gets booted out. To revenge himself on her, Deslauriers tells her that Frederick is getting married.
And guess what? That's when Madame Arnoux realizes that she loves him.
Back in the countryside, Frederick and Louise are taking a stroll in the garden. Louise confesses her love and asks him to marry her. He rather unenthusiastically consents.
That's right—he's getting married.
He gets besieged by letters: the Dambreuses asking him to dinner, Deslauriers telling him to stay away from Paris, and Rosanette thanking him for risking his life for her (she majorly misinterpreted the duel) and asking for a loan.
He gets out of there, feeling the heat from Louise about getting engaged.