I'd say that, and just about everything in the concluding chapter, would be the "Return with the Elixir" stage of the hero's journey.
Though thinking about Eb as a mentor, he would probably qualify in Proven Guilty when he convinces Harry to figure out what's going on in the Faerie Courts.
But this doesn't really help the argument that as a mentor, Eb has to die because many characters act as a mentor at some point. I'm sure we could fit Michael, Lea, Thomas, Maggie (Sr.), Malcolm, Rashid, Eb, Bob, Odin/Vadderung/Kringle, and Mab in there. We can probably make decent arguments for Uriel, Murphy, Butters, Murphy's dad, Mort, Sir Stuart, Carmichael, Billy, Luccio, Molly, and probably some others I haven't thought of. I don't think they are all going to die just so Harry's "hero's journey" can progress.