Meaning: he had to let Harry know where Hermione was going to be, and how much danger she was going to be in. Hermione had no way of knowing where Harry was, but Voldemort must have thought that their friendship would have resulted in some kind of correspondence. Volatile, hot-tempered, loyal Ron would move heaven and earth to rescue her, that much she knew. If they knew Hermione was in trouble, real trouble, actual trouble, they would come for her. 