Diane considers telling him that he had help with that, then decides against it. While calming him down might have been a bit manipulative, it hadn't been the least bit invasive... but she remembers how huffy he got when he thought she might be in his head, so she's not about to tell him she might have been improving his mood without his knowledge.

Anyway, it's not as if he hasn't quickly reverted to being his usual, somewhat grating self. It'd take more energy than she'd care to expend to try and coax him into a better frame of mind all the time.

"The next person you crash into might not be," she points out instead. It won't matter how calm he is if his unwitting victim panics - and given what many two-leggers think of bats, they might (even if they're in animal shape).

Once he takes off, she follows him, ready to intervene if things start to go drastically wrong.