But she wasn't half naked.
Or was it?
It is unlikely that we will know for sure, because it was his fantasy.
If you didn't understand, then all the pictures whose edges are covered with a white haze are the fantasy of the main character. Pay attention, when he talks to his mom, there is no haze around her. The MC's mom hands the phone to Kelly, and now she's already sitting on his mom's desk, shrouded in a haze, and Linda, sitting behind her, has disappeared.
So considering that Kelly has an incredibly sexy voice, it's not surprising that the MC presented her half naked.