Sexual assault is a crime, but a citizen may not be able to file a criminal complaint themselves (as far as I know you cannot in Washington for felonies, including sexual assault). But for the sake of argument, here is the US courts criminal complaint form. Notice that you are required to give your name and swear to the truth of the accusations. So it is impossible to file an official criminal complaint anonymously. (The form does not mean that a citizen could file a criminal complaint in US courts: it means that the person filing the complaint must identify himself). However, you can anonymously describe the facts to the police or the prosecutor, which could lead to an official complaint. Incidentally, if you believe that the person actually committed rape, notifying HR is not an appropriate response.
Given what you describe, there was no sexual assault. An employee saying something like that could constitute "having a hostile workplace", with respect to sex, and the company could get sued. There is no law whatsoever pertaining to a person alerting HR to such a problem. Instead, it would all be a matter of company policy. It is possible that company policy precludes accepting anonymous complaints of inappropriate workplace behavior. The consequences of such a policy could be costly.
Under EEOC rules, sexually discriminatory statements are sexually discriminatory statements, no matter who they are said to. The company is expected to take action if it know, or should know, about the conduct. By refusing to accept a complaint (anonymous or otherwise), the company could be mistakenly thinking that they are protected if they don't actually know.
You yourself might file a complaint with EEOC, or with HR, but no official legal complaint can be anonymous. You will eventually have publicly accuse the wrongdoer. You can anonymously report the facts to HR, and they can ignore that information at their peril, if someone later files a formal complaint. Once you've ignored notification, you're in the territory of "should have known" .