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I've never heard of a pentacle with a devil's head in it being a particular symbol of any group or tradition. Does she mean maybe this instead? http://en.wikipedia.org/wiki/Sigil_of_Baphomet

The Sigil of Baphomet is pretty stereotypical as a Satanist symbol, but it doesn't really represent evil. Although if a person believes Satanism and paganism and witchcraft are all the same thing and it's all evil, you wouldn't convince them of that easily.

I'd assume she's seeking attention, more than anything magical. Give her an amulet or a stone or something protective, something that can ward off unwanted entities or magic. (Or hell, a placebo, since she likely won't know.) Take a very reasonable, rational step - and then see if she says it doesn't work, or if her 'visions' escalate into another form of harassment. If it does, chances are she's just looking for attention.

For what it's worth, I agree with Morg. I know a lot of attention-getters around me (well...used to be around. Hooray for shedding dead weight!) that were all about that particular sigil and how evil it was and it must be the devil. In the end it was all eye-roll worthy.

Poor Baphoment.

But I'd go with what Morg says, and probably start with a placebo. If things stay the same, it's possible she's being somewhat sincere (somewhat) but if they escalate or are suddenly better, well, you know what's what then.