And if she made a false claim based on misinterpretation she just ruined an innocent persons life. I have seen people that were cleared of all charges but it was to late to save their lively hood and sadly even their marriage.
And if he did nothing wrong then he has nothing to worry about. Making a report ≠ automatic guilt and conviction.
In regard to what I quoted, from what I have observed, all it takes is one person to say you did it. Even if there are multiple students claiming this, how can you know they just aren't a part of the misinterpretation? In Jr. High, I was accused of racism, totally innocent of it, and you wouldn't believe how much people I didn't know, never seen before, heard of before, blah blah blah, who would say they witnessed me saying something racist. Just looking at them I could actually believe them, as if I was the obvious bad guy you see in the movies, and I didn't even do it! This one thing, which I never said, lead to many death threats and years of harrassment. I have physical scars from it. It ruined the midteen portion of my life and my (two) brothers' lives, and was solved by switching schools and staying low (problems occasionally occurred outside of school, but it eventually died down after a couple of years).
This sucks cause there aren't too many options for you to do. From my own experience, I'd approach him first since I consider losing anonymity less of a loss than the damage that could be done to an innoncent guy (value is subjective though, you may consider anonymity more valuable than what you think are the possible, or likely, damage to this guy), but if you choose to report it instead, report with extreme caution. Be aware of what you're doing here; the possible repercussions of reporting. I don't think this guy would get beat like I did, but I am sure others would join to make sure "this guy gets what he deserves" from people I'd imagine to be like bstone (please take no offense to this, just from the comments I've been reading within this post, I'd imagine you to do something like that, you're already practically leading an anti-this guy campaign on this forum without witnessing it or anything, you are just taking her word for it on everything she has said so far). He could lose his job, probably never get a job in this field ever again, etc. Don't "I think he said this", or anything like that. If you can replay it through your head, and it can change, I'd probably not report it. I think this situation could be just like the whole Emch thing we read with two flight nurses performing a cricoidectomy. This is harder than that Emch thing though since it's one of the "he said she said" sort of thing almost. I doubt there is any real clear cut evidence like audio and video recording for this which would quickly tell people "Okay, this is what he did wrong, and this is the punishment he deserves".
I hope that everything is dealt with appropriately.