(1) For the person who posted the name of the person who raped them: I'm also not a lawyer but you have a few options. The first would be to take it down. If you want to leave it up, I would contact your local rape crisis center. Hopefully they have legal staff who could help you for free, but even if they don't they would at least have advocates who might be able to help you navigate this. The thing is, people can sue or threaten to sue for whatever they want.
(2) Whether they would be successful or not is another question. Part of whether they would be successful or not rests in whether or not there was any kind of conviction and what the circumstances were of your posting. If he’s been found guilty in any capacity, then the law would be likely to be more on your side than if not. And if the purpose of your post was more about alerting people to help keep you safe a jury might have more sympathy than if it was a post aimed at tearing him down.
Ultimately you need to weigh if this is important enough to you to fight it. It’s possible they will go forward and a judge will throw it out. It’s possible it would go forward and you could have to pay damages. It’s possible they are bluffing. So weighing those possible outcomes you need to decide if any of those outcomes are ok with you. I’m so sorry that this is going on for you right now and I hope that your local rape crisis center has a lawyer you can talk to about more specific questions.