Talk about a last-minute addition: Monsieur Bonnac's got one purpose, and one purpose alone. He's there to show how generous and wonderful Valancourt can be, even when he seems like he's being a bit spacey. But Bonnac's made a wreck of his own life, so he knows how miserable it can be to have lost it all. He's thrilled to have been bailed out of jail, but he wants to make sure to properly thank Valancourt.
Oh, and Madame Bonnac's got a sweet deal: she's the nearest living relation of the house Udolpho, meaning that Em's got someone to dump an unwanted castle on. It's cool how both Em and Valancourt can help out the Bonnacs in different ways—kind of the like the weirdest double-date ever.