I agree, genes rule. I disagree, Blind Guardian is in their own category. Just listen to the song And Then There Was Silence. That song is amazing and no other band can do it, nor write it.
I think you can't get into metal like that, not by listening to certain albums "Now this album is death metal, that album is power metal..." If your friend listens to a lot of metal he'll eventually find out the similarities between bands and figure out what the subgenres sound like. Anyway, enjoying the music is more important than being able to lable it.