It was a believable lie because it was entirely based in fact. Every single second of the day she worried about her son. She hoped that he was all right in the castle. If he tried to send her an owl, she worried that either he wouldn't know where to send it or Rodolphus might intercept it before she received it. At the mention of Oliver, Rodolphus grew noticeably chilly. He had never been a fan of any children at all, only giving the minimum amount of attention to his nieces and nephews required to be less than an ogre. The fact that he had never been able to have children didn't bring him the least bit of sorrow. Oliver was not a subject the wizard cared to be reminded of.