I won't speak to the legality of it because I'm not qualified, but it's morally reprehensible.
He could have
1. Taken offense to this comment/taken interest in what else this user comments (which you can do without any admin tools)
2. Seen the islamophobic/xenophobic comment
3. De-anonymized the user
In this version the de-anonymization is related to the more repugnant comments, not the initial critique.
Take the internet away and it's even more obviously protected. If you ran a newspaper and occasionally published letters from readers, and if a racist letter showed up with a vaguely obfuscated address that was easily traceable, it would be well within your rights to write an article about the person in a position of power who sent your newspaper racist rantings.
The journalist likely violated Disqus's terms of service but in no way does that give the professor grounds to sue him.
I've been surfing everything from Usenet to webboards and other internet comment facilities for 25 years, how come I've never seen it? Typically the routine goes: cosmetic blocking (hiding, disemvoweling), account block, email block, ip block, then whack-a-mole for the truly persistently annoying.
Have you, and can you link to it if so?
There's a reason certain forms of evidence are inadmissible in court.
(The professor's also at fault for not anonymizing herself better, of course.)