Venting anger helps. Often when I was pissed off at my client I wrote angry letter pinpointing all the inadequacies in his behavior. After I wrote it I was calm enough to write another letter where I proposed constructive ways to alleviate the problems we had.

Of course I was just sending the second letter because only useful purpose of the first one was to help me calm down.

If you felt that strongly, then letting it out is the same as letting it go. Psychologically, it probably set him properly for his next role.

Good on him for not cowing under the "business norms" here.

And as to hurting his reputation: that sounds like you're referring to rep to upper management, who will probably not care, and will have hardly anything to do with his next hire, anyway.

People who will be choosing to hire him are pretty likely to identify with him at some level, even if it's historically, for a start. If they'd be worried about his "honesty", he really wouldn't be happy working there, anyway. In my experience, people who've been totally honest about their opinions to colleagues and managers have been, on average, more successful and definitely more respected.

Then again, perhaps that's just a antipodean perspective.

Here's a more interesting scenario: assume that I own a significant amount of stock options from that company. Don't I have an incentive to make sure that the company succeeds (even though they fired me) and therefore try to explain to management what the current problems are?

I think he's helped his reputation. I'd hire him.