In the Inferno the lowest circle of Dante's Hell was reserved for the traitors (Ciardi refers to them as traitors "against those to whom they were bound by special ties" [p.266]). It is sub-divided into 4 parts: Caina, for traitors against family; Antenora, for traitors against country; Ptolomea, for traitors to guests; and Judecca, traitors to benefactors. Brutus and Cassius appear not only in the lowest part of the lowest circle of Hell, but in the very mouths of Lucifer himself, alongside Judas Iscariot. Dante saw these four (Satan included) as the "arch-traitors" (Carroll). In the context of the poem, then, Dante has placed Brutus, Cassius, and Judas in the mouths of Satan because they are the worst of traitors against their benefactors/masters: Brutus and Cassius killed Julius Caesar, who was their rightful ruler; and of course Judas betrayed Jesus. In this way Dante has chosen sinners who represent treachery against the two highest authorities, the Empire (Caesar) and the Church (Jesus).

In reference to your question, it is true that the reason that Dante placed Brutus and Cassius where he did was because they betrayed Caesar. I think it however it would be incorrect, or at best a stretch, to say he does so because of the friendship between the three men. Carroll in his notes talks about some of the distinctions between the different sections of the ninth circle in more detail; he talks about the third section, Ptolomea, as including traitors to friends as well as guests, which Dante places as a sin second only to traitors to lords and benefactors (so this is something Forster does get right: Dante saw treachery against friends as worse than treachery against country). But there are a few counts where this idea of Forster's falls apart, the main one being that Brutus and Cassius are not in the section of Hell for traitors to friends, but for traitors to masters. (Another point is that I think it is questionable as to whether Caesar and Cassius were 'friends' but I don't really have a way to back that up one way or the other). Although Dante does not spend much time in the poem discussing these sinners (they move quickly out of Hell after seeing them) it is clear that Dante is drawing a parallel between the betrayals of Judas and Brutus/Cassius, as the paramount sins against Church and Empire respectively.

The text of Carroll's notes (for Canto 32) can be found at the Dartmouth Dante Project, http://Dante.Dartmouth.EDU. I use the Signet Classic (June 2001) edition of John Ciardi's translation of Dante, which is my preferred translation.

It's more complicated than that. Although you could view the betrayal of friendship as playing a part in their fate, it was more their betrayal of a fatherly figure, or their failure as filial subjects.

Dante had a soft spot for monarchs. Just read De Monarchia, in which he argues for a universal empire. In fact, he put Henry VII of the Holy Roman Empire with his own throne in Paradiso because he believed he would bring unification and peace to a fractured Italy.

Follow up question please.

Does Dante's Inferno have any kind of claim to religious legitimacy? Not as in is there a god or a heaven and a hell but is it based on prior Christian theological teachings or is it just a poem that Dante came up with?

Does anybody believe the Dante was right and his description is what hell is actually like?

Thank you