I think the University is within its rights to fire him from his administrative position for public back-sassing, even if he's absolutely right and speaking out of concern for the school.
But stripping him of tenure is a whole other animal. The bargain that faculty administrators make with their school, when they retain (or are granted) tenure in their home departments, is this: I will stop doing the thing I'm trained for, and take up this service role, and in exchange, I will be allowed to go back to it when you're done with me.
When a dean (for example) is hired from another university, the department that would grant tenure has an actual discussion about whether they want the person hanging around on their faculty if things go wrong. It can actually derail a candidate even when the administration really wants to hire someone.
Since he'd never be fired from a tenured position (without a gigantic outcry such as is happening here) for criticizing the administration if he weren't a dean, this is pretty bad.