While rape is a very serious charge, date rape
is pretty difficult for the prosecutor to prove. Not that forcible rape can't ever occur between people who have longstanding friendships or relationships, but a prosecutor has to prove the charge beyond a reasonable doubt, and when two people have known each other for years it's a very heavy burden.
Additionally, from what you're saying, she has not even gone to the authorities yet. That would mean that no rape kit was taken from her, and apart from her word, there is no outside proof that they ever slept together at all. And, of course, he has a consent defense and you are saving their correspondence to show the development of a close relationship.
If she goes to the police and your son is contacted by the police and told that they want to speak to him, he should not go. If he is the suspect in a criminal
case, he does not have to cooperate in what may result in his own arrest. He should let it be known that he first wants to speak to his lawyer, and then he should start looking for a criminal lawyer just in case.
He should also talk to nobody about this incident, especially not the woman who may accuse him and their mutual friends. Anything he communicates to anyone other than his own attorney can be used against him if she does report the offense and takes him to court