he can claim that she also blackmailed him by threatening to talk to his parents.
Its not illegal to tell people the truth...i believe it only gets illegal if she asks for money or favours NOT to tell them , which is not the case here...She needs to tell the Police , so that if anything happens they have some history on the situation , and perhaps warn him what the result of carrying out the threat would be...Perhaps knowing the Police are aware of his threat would help him to realise she is not going to be a helpless victim...He is a bully , and the one thing a bully really can,t deal with...is somebody who is going to put up a fight...
I have had experiance of this kind of person before , and in every case the only way to stop it was by making their lives more unhappy than they are making yours...Even a sociopath gets tired of being arrested , spoken to by the Police , and as a last resort getting their face punched in....Not that i am advising a woman alone to get involved in a punch up with a guy..she will lose...in her case the only solution is to get out of the situation by whatever means , and keep the guy at a distance by legal or otherwise actions....