The student was obviously a jerk, but still within her rights. It was simply a failed negotiation. And it could have depended on many factors we don't know. Since wearing a mask was an elective act, many other factors could have played a role. Maybe the student was just an entitled brat. Maybe the professor was just not very good. Maybe he had refused in the past to accomodate students in other ways. We don't know. But what we know is not enough to make definite pronouncements either way.
Ostensibly, he (and probably other staff) had voiced their concerns to the university and the university still decided not to require masks. One might disagree with that decision, but that doesn't give people who disagree with it the right to dictate their own rules.
Finally, it's pretty clear that the professor was already looking for a way out and found a trigger he could pull:
> "I felt some relief as I had been getting more concerned as the COVID-19 pandemic unfolded in recent weeks."
He was asking them to make a reciprocal gesture of respect.
Deliberately wearing a mask as to render it ineffective in the circumstance is deeply disrespectful.
It’s a pity one student ruined it for the whole class, but upon reflection maybe a more important lesson will be learned by all.
I think the real concern here should be the harm done to the other students, both by the one student who couldn’t be bothered to put on a mask and the fact that they have now lost their professor.