WOW. Joseph Pearce passed my question (see post below) along to Tony Esolen and I got the following reply:
Dear Celine,
That's an interesting question, and I'll venture down a couple of avenues to reply.
In Canto 17 (if I remember right) of Paradise, Dante's great-great-grandfather, Cacciaguida, gives us the practical justification for Dante's naming and praising and blaming people who are prominent in public life. It's that mankind will only pay attention when the name is famous; and C. S. Lewis, in The Great Divorce, picks up on that in an interesting way, by placing well-known warmongers and troublers of the peace, such as Napoleon and Henry V (so as to balance a Frenchman with an Englishman, I think) in the farthest reaches of the Gray City below. Virgil had done something similar in the Aeneid. These are, of course, figures from the past.
Now, what about people who are still alive, such as Boniface VIII? I think that Dante leaves himself open for criticism there, since he doesn't imply that Boniface has even a chance of saving himself -- he WILL be in Hell, and so will Clement V. That may be (well, it is) presumptuous, but of course Dante knows that you know that he is writing a poem and NOT pretending to speak for God. Then the question becomes whether he is committing the sin of detraction. That depends. Suppose you are in possession of knowledge that person X has done something gravely evil. You have a duty to reveal it to the authorities if the deed in question is a crime against the state and the common good (rather than, let us say, an act of adultery which violates the moral law but is not by itself an attack on the public). You also have a duty to reveal it if the person is a public figure and the crime violates the trust he assumes by his position of authority. So if you know that Hillary Clinton has traded state favors to foreign nations in exchange for campaign money, you are not committing detraction if you reveal it. Supposing then that Dante is certain of his information regarding Boniface's collusion with Guido da Montefeltro, it is no sin to reveal it, and it may be sinful to conceal it.
We then should ask why Dante thinks he is so certain about that act of treachery. Here I can't answer. No one knows. If he is not certain about it but is repeating hearsay, or is guessing, then he is guilty of presenting something as certain which is not so.
As for the reputation of Guido da Montefeltro, again our evaluation of the morality of Dante's exposure of the man depends upon a question of fact. If in fact he did what Dante alleges, since he is dead and thus can suffer no consequences from the loss of his undeserved reputation for holiness, the act is not one of detraction; and it would protect the common people from a mistaken reverence toward someone who did not deserve it.
That's as I see it. On the general charge of Dante's being judgmental, I think we can say that he reminds us that this life is not a game we play, and if we lose we go home and everything is all right. The stakes are infinite.
I hope that helps,
Tony Esolen