If she hasn't broken the law and hasn't made any threats to harm someone, what are you going to tell the police? "The crazy lady bought a gun!" I don't they can do anything unless a crime has been committed or is contemplated.
At this point, the only person who can take action with such information is the spouse. He may be in danger and, with such foreknowledge, decide now is the time to put some distance between him and this marriage. Or he may buy a gun, too, just to level the playing field, and one or both end up shot and/or dead.
It is a conundrum. A moral dilemma. I understand your struggle with it, but don't know that you can do anything about it.
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Just another self-proclaimed expert ...