well, aluminum will bind very quickly with fluoride, forming aluminum flouride, which is just about the LAST compound you would ever want to ingest, as the fluoride will serve to escort the aluminum into your brain, which puts you at great risk of alzheimers and all kinds of other disorders. the fact that your friends hair tested high for aluminum illustrates this quite well.
but you are probably right about cities using alumina filters.
both boron and calcium bind readily with fluoride. i am sure the bone char is high in calcium which would mean it does leach out some fluoride, but exactly how much i would have no idea. i am really not a big fan of ingesting boron in the form of borax, but it may be less toxic to add a tiny pinch of borax to the water to bind the fluoride than to leave the fluoride in there. just a thought.