"Did you know you'd get something out of it before you told him?" Theodore asked. Neville didn't seem like the type to tell Rosier that he'd been helped and try to bargain, although he could have. That seemed a little stupid to Theodore. He was surprised enough that a bargain like that actually worked; he wouldn't have expected Rosier to let up so easily, not even for the name of someone who had betrayed him.
Although Theodore hadn't done it with the intention of betraying Rosier, not directly. If he'd intentionally betrayed anyone, it was his father. "He isn't stupid. I must have gone too far, improved your condition enough for him to notice."