The FFVII game version or the Advent Children version? both are awesome
That's an unfortunate situation, I slowly progressed, and made my mum listen to the kind of music that I like (I enjoy all kinds of music, I'm just talking about the ones that she's not accustomed to). Rock, heavy metal, gothic, orchastrael, "Epic Music" (E.S Posthumus, Immediatte music, X ray dog, Two steps from hell).
Over time she has begun to like them and understand them better, especially when I explain the songs meanings, which are usually much more deep and emotional than most of the contemporary music these days.
No offense, but your mum just seems to be over-reacting, I have also let my mum listen to One Winged Angel (Both versions), and she liked them. They fit the mood of the game and movie greatly, and even when she just listened to the music alone, she still agreed that it was beautiful in its complexity and variety. If you have a desperate or frightful situation in a movie or game, then the music has to also be consistant with that tone. If the music doesn't match than whats the point? It certainly doesn't make it demonic, at least my mum doesn't think so.