I don't see why this individual should not turn himself into the police, at least if he is incapable of restraining himself, if he has committed a crime he should pay the applicable penalty. In any event, assuming the person is incapable of making restitution and of genuinely repenting of his sin, which involves an effort to stop committing it, he has already cut himself off from the sacraments.
I don't know what the OP's intended meaning was, but the way I read it I took Mr. Crane to be an older gentleman who, while mentoring the 13-year-old Emily, ended up falling into disgusting and totally-inappropriate sins of lust with her. He eventually came to his senses, stopped committing the sin, repented of it, and went to confession. There is no indication that he wasn't sincere, nor is there any indication that he is incapable of restraining himself, especially after having received the grace of the confession. If he were a truly wicked and unrepentant man who will never be able to restrain himself then perhaps he should be locked up, in order to protect at least his future victims from his sin. But if that's not the case, there's no point in destroying his life or in tormenting him with sins already forgiven.
On second thought, I guess this is probably not what the OP had in mind. Because if this were the case, what would be the point in the priest warning Emily's parents about something that was no longer an issue? Could just be that he's being extra cautious though.
The stage of sexual development of the victim makes a difference to the precise term used to describe the perversion in question. (A thirteen-year-old's development will vary by individual.) Legally and morally it is sexual abuse of a minor. Mr. Crane has molested a child in this scenario, just as if he had preyed on an eight-year. It is not a difference that makes a difference.
There is a difference though. If your point is only that Mr. Crane was trying to take advantage the weak and defenseless, then I concede the point. It may have been so.
However, we don't know the reason why he went after her. Maybe he was "preying" on her, as you say. Perhaps he sought the mentorship just so he could be a pervert. But there is another possibility: Maybe he honestly wanted to help out at the parish / get involved with mentoring, and, in the process, unfortunately ended up falling into a sin of acts of lust which merely happened to be directed at somebody who happened to be a defenseless minor (the same could have happened had Emily been, say, 16 or 17 years old, not as defenseless).