In between the time that she felt 'get this thing out of my house' and the time she found that it was actually still in her house, did she have any of these problems? If not, then it doesn't seem as if the existence of the necklace itself is the problem. While it was lying dormant in a drawer it did nothing. If it was only when your friend buried it and and asked forgiveness (the bones may have been perfectly content to be a necklace - not everyone is squicked out by that kind of memorial) that her problems started it may be that that rejection was the catalyst.
Finding out what such a necklace means is the most pressing priority, I would think. If it is a guardian or luck thing, it may have been seriously offended at being 'thrown away' and apologized to. Rejecting a protective talisman can conceivably cause it to become hostile. Maybe another apology is in order, if she still knows where she buried it. And then maybe she can pass it along to someone who would appreciate that sort of thing or return it to Guest, who may have simply forgotten it.
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