If he's quick witted enough to insult someone based on their physical attributes, I guess he should've been quick thinking enough to grab the lady moaning loudly behind him.
Besides, you're exaggerating quite a bit, aren't you? To other people it would probably look like he was just watching or listening to something with adult content in it and forgot to turn it off or something, which is pretty understandable. I mean, who here has NEVER watched or listened to any media with adult content in it?
Also, there are invisible people in their world, AND an invisible girl in their class. Anyone who has heard the invisible girl talk would be able to match the voices and know that it was her, so if he was really worried about his future he could just tell on her.
At worst, this is about as silly of a prank as when you pick up the phone and your friend starts moaning in it or something when you switch it to speaker.
Besides, with how scummy this dude acts, I GUARANTEE he has ruined his own reputation way worse than she ever will.