Some people are impressionable. Very impressionable. And some Christians can get pretty defensive (mostly the old, old, old ones). People as a whole tend to judge something based on ignorance, then twist the information to support their point.
Christians aren't perfect, nor do they claim to be. Some people made a mistake driven by defensive but ignorant action, and it's still kind of around.
But she is wrong. The thing about D&D is that it allows for such freedom that if someone wanted to incorporate their Satan worship into the game, they can. It does not mean that it is incorporated into D&D itself, because it simply is not.
And no, not all Christians are like that. In fact, most aren't. They aren't nearly as closed-minded as everyone thinks.