So I guess his argument is that every game is now retconned to accomodate the fanservice fluff and retroactively no character has ever died. So unfortunately I don't think that would break through to him.
Give it time. With enough evidence, he'll probably crumble in a heap of deniability when he see he's wrong.
As for my take on Awakening...As far as I know, the game itself is canon; it is implied that Awakening takes place in the same timeline as Marth's adventures (FE1 and 3, for those who don't know). I also heard that the Tellius games (PoR and RD) are also in this timeline, but I need to confirm that for myself. The Spotpass DLC, to me at least, feels like an extension of the Netherealm Portal DLC in that the characters you actually fight (and fight alongside) aren't the actual heroes themselves, but just wandering phantoms. That's probably why they're all still "alive," even though certain characters like Fado and Hector are clearly dead in canon.
To me, all the DLC is canon in the same sense that the One Piece movies are canon...They happened, yes, but they don't really have much of an impact on the story, they're just a fun side adventure.
I dunno if my two cents will help at all, but that's my opinion for ya.