There’s a lot to choose from, Nigel. There was no
Divine Comedy in English until nearly 500 years after Dante’s death, early 19th century, but there have been
a slew since then.
I disagree that terza rima is the most important thing to bring over in a translation of the
Divine Comedy. Dante’s language is an inimitable mix of earthiness, formality, colloquialness, intellectuality, bluntness, precision, and inventiveness (when he doesn’t have a word for something, he makes it up). Doing the translation in terza rima is certain to cancel out several of these at once, leaving mostly the rhyme pattern, which is so much easier in Italian anyway. So, my favorite translations of the DC are the blank verse ones, though Ciardi’s defective terza rima (rhyming only the first and third lines of each tercet) is a perennial favorite for many. Robert Pinsky translated only the
Inferno, using off-rhyme effectively, and I like his version quite a bit.
For the whole DC in translation, my preferences are Ciardi, Mark Musa, and Robin Kirkpatrick. This last one is published by Penguin in a deluxe edition, with parallel text. The Ciardi is not bilingual. I think the Musa is. Kirkpatrick’s also has an excellent commentary. A lot of people like the Hollander version (Robert Hollander died just a few months ago), which is also bilingual.
I don’t know what your budget is like for this, but since your friend’s birthday is such a special one, if you can swing it you might consider
this new bilingual edition, just published by Polistampa in Florence, under the auspices of the Società Dantesca (for the 700th anniversary of Dante's death). It’s a two-volume beautifully produced edition of all of Dante’s works, in both the original and facing-page English. They are using my
Vita nova translation for it, as part of volume 1 which includes all of Dante’s works besides the DC. Vol. 2 is the DC, though since I haven’t yet received my copy I’m not sure which translation they chose for that: I am guessing the Hollander.
For the beauty and novelty of it, this edition might do the trick. If not that, I’d go for a hardcover edition of the Kirkpatrick or the Hollander.
Good luck.