This is actually a very interesting and complicated issue.
The guy's thoughts definitely aren't healthy. Everyone thinks things they don't mean (even Mana in this very series), but he's apparently reveling in the thought of molesting a girl and making her cry, which is taking things to another level. People who spin such long and detailed trains of thought about terrible things are certainly closer to actually doing them than those who do not.
On the other hand, no matter the length or detail, thoughts are definitely not considered a crime. Preparing to commit a crime would be him physically reaching out towards her, not merely contemplating the idea. Let's not forget that controlling one's thoughts is a lot harder than regulating one's actions, especially when someone's tired or is under other types of mental stress. There even exist therapies involving shouting your grievances into an empty, sound-proof room, with the obvious assumption that even if some nasty words are said about someone, they'll never hear it. This guy would, unsurprisingly, never consider the possibility that the girl in front of him is a telepath; if she would merely look him in the eye right now (I'm not saying she can or should though, she's too terrified to begin with), that would more than likely shut him up for good.
Also, we must consider once again how Nakano's powers work in situations like this. She usually hears/sees the thoughts of others as if they were talking, right? But thoughts do not actually work that way. For instance, if in a burst of anger, I think "Man, what would I give to punch your face in a few times!", even while this thought forms inside my head, an "undercurrent", a feeling of guilt joins it almost immediately. Even as I think these thoughts, deep down I know they are not right, and know that actually doing this would accomplish nothing. Thing is, however, that as far as we can tell, these aspects of one's thoughts are NOT picked up by Nakano's powers, especially when they are not accompanied by any sort of imagery. The guy in question may or may not actually be as enthusiastic about molesting her as his surface thoughts would suggest. Disturbingly enough, we might never know until he actually tries to go through with the deed.