Lawsuit
The thing about being sued is that it can cost you 100K, even if you win. Make sure the publisher's lawyers vet the book, and even then, you can almost bet there will be a clause in your contract that says you're on your own.
Truth is enough to let you win, but opinon and truth get mixed up often.
And legally speaking, fifty opinions carry no more weight than one opinion. You can probably get away with writing the bare bone facts of what this person actually did, but if you let opinion of what these things made him look like slip into the writing, you could be in hot water.
It's a fine line, but if this person is simply an individual, not a celebrity or a crminal, procede with caution. He can certainly sue, and can certainly cost you a LOT of money, even if he loses. And if he isn't a celebrity or a criminal, he just might win.