She is not obligated to go to the police. She can, in fact, just tell the Internet what she believes happened, and tell him to go fuck himself if he has a problem with it. If she's lying, he can sue for libel.
Well, considering that he is innocent until proven guilty, he can sue for libel until her charges have been proven.
This is not correct. In the United States you must prove an intent to defame. Sure he can sue for liable, but he wont win unless he can prove intent. Even if the charges against him are not proven. This may seem harsh to you, but read about liable in the UK to see what a train wreck the other alternative is.
Accusing someone of sexual assault is defamatory. This would be a clear cut case.
You're not following. The intent must be malicious. That the impact is defamatory is, at least to some extent, besides the point. What's he's saying is that she could even be mistaken and still not be liable.