And even if you are too scared, and want to report it without letting people know who you are, thats a start.
I wrote a letter to the judge. When I researched how anonomous letters are treated, there is no standard procedure. I've read some judges reading them and acting on them, treating as an anonomous tip, and checking it out, to judges ignoring the letter, or passing it out to both the plaintiff and defendant.
Cops gets anonomous tips all the time. They may put it in a field report, which can be later reviewed in the event of more crimes with actual evidence.