Hooboy.
People like this generally have a limited scope of what’s right and wrong–if it falls outside a biblical parameter, it’s considered taboo. It is, I think, based on a fear factor–if you expose yourself to anything outside your tiny conception of the world, the world is a lot bigger and scarier, and–gasp–things which you held dear may not be correct. There’s pride stepping in, which, BTW, is a biblical no-no.
I’ve always felt that one of the purposes of arts and entertainment is to vicariously expose persons to viewpoints and experiences that they themselves can’t or haven’t yet experienced, in order to deepen one’s understanding of the human mystery. For example, I haven’t had kids yet–hence I have no personal experience–but I may get an idea of the meaning of fatherhood and how I should behave based on some character’s experiences in a book, if that’s what the author seeks to portray.
Now, moving into the specifics–there’s basic concepts in Harry Potter, Pokemon, etc–that deal with virtues which any decent person WOULD want to develop. Let’s take Star Wars, for example. Luke Skywalker is an “everyman” sort of character–the audience can relate to his wanderlust, his eagerness, his anger, etc. He’s also got a lot of “normal” flaws: he’s too much of a dreamer, too controlled by his passions, too disobedient, and all of these things do come back to haunt him with real consequences that he caused. There’s valuable lessons that the audience can learn about themselves and carry into life specifically because we can identify Luke’s flaws in ourselves.
The Bible doesn’t quite teach these things, largely. Because it’s written in an older narrative style, it’s not as easy to relate to the stories (although there’s virtue value in the stories of David & Bathsheba, Daniel in the lion’s den, Christ’s courage at his execution, etc). Although it teaches Christians their history, their destiny, and what rules to follow, it doesn’t necessarily relate to the individual per se. Hence, outside literature and entertainment can have value as supplementary material. (And it’s cool.)
So if mom is freaking the hell out, you might ask her to walk through one of these things with you. Say, for example, watch a Pokemon episode together or rent LOTR when it comes out next week. Have a discussion afterwards about the merits and flaws of the program; see what a good Christian should take from the show and what he shouldn’t. Talk about the virtues and vices of the characters (keep away from the “incidentals”–Luke Skywalker is using magic! Bad! Bad!) and see what happens.