system of a down is FAR from devil music. have her listen to something HEAVY.
go find "bring me the horizon", and have her listen to some of that.
its not devil music, but its A LOT heavier than S.O.D.
then she'll be glad youre not listening to that, and she'll have no problem with S.O.D.
same with some other stuff like "clifton", "all out war", and others, now of course those arent the heaviest stuff around, but they are up there.
i dont think there is really such a thing as devil music, but manson is as close as it comes i think.