A place for discussing the differences between dialects, I’ll start with a question of my own:
If Ἄϊδι is the Epic form (Il.1.3) why is Ἅιδι the Attic form? I thought that τῷ ᾍδῃ would be the Attic form. Is Ἅιδι derived from a slightly different form of the epithet?
ᾍδῃ shows up in Euripides sometimes, at least. I don’t think that Ἅιδι is real. If ΑΙΔΙ did show up in Attic outside the context of a quote from Homer, etc., it would be impossible to tell the breathing anyway, without some statement from a grammarian.
It’s just another place to check when what you’re reading doesn’t make sense. Reading your question and Joel’s answers, I went to the Thesaurus Linguae Graecae to see if Ἄϊδι is used anywhere other than Homer and failing that, I found the corrigenda for Learning to Read Greek on the Yale University Press website and sure enough, there was the error. Most things aren’t perfect and textbooks can have their share of errors as well.