It’s perfectly fine to let her sit there and twiddle her thumbs. Truly. Even as an adult, there were some classes I had to take, but was not that interested in. Would you say it was my professor’s...
Exactly. A professor's job is to teach the material, not to entertain or cajole. The student's responsibility is to show up and engage. If she chooses to disengage, that's her call, and it's fine to let her sit with that choice.