and word from On High is he will never be head again.
A. Don't believe hollow comments. B. A department head is just 1 guy and its certain that others were involved. C. Environment will suck bad for him and will be toxic for everyone. D. The turd with tenure that deserves to be fired cannot be fired. E. You will have to be stuck with the idiot.
Problem I see is that even if he wins his case it's gonna be bittersweet and uncomfortable to stay. I think it's time to tell the department it's their loss and leave. Winning the case and staying doesn't incentivize anyway to act differently in the future, leaving does.
This is probably the best solution. But I'd say that he should make his case just for name's sake. Then both of you should make your case at other universities as he would not want to stay there and now that you know what you know you know that they are going to be mean to you. Maybe both of you will get a job at a better place. At some point
The problem with tenure is that the system is broken. If somebody is a bad colleague you can't fire him like in a normal firm. Worse the guy can be there forever and there is no way to lock him in the dungeon and hide him from the world. Its about time people take a stance against these cases and if the environment is bad, move and don't be shy to tell the dean or EJMR that the environment is bad because of one guy. In particular, you don't need to tell the dean or tell anyone in real life, just come here and vent the dirty laundry and then mail the dean with some anonymous mail pointing to EJMR. It has been working because deans don't want bad publicity.
But above all, you might want to move with your colleague to some other place and he should challenge even if its just for his pride and there is no harm in having a few more offers just to see what people think of you.