What do you want them to do? I suppose that they could go talk to him, but is that the best way for them to spend their time? Should they go talk to everyone who has made similar comments? Should they talk to everyone who is accused of making similar comments? Should they keep files on all of these people?
I know it sounds callous, but it would be impractical and a serious infringement on all of our rights if they did much more than nothing at all. After all, he could just as easily go to the police and claim that she told him that she's a drug dealer and that she kills prostitutes and feeds them to her dogs.
It *is* his word against hers, because they have nothing else to go on. They don't know anything about the situation.
If he is abusive, she can press charges for that; ditto threatening to kill her. She can also leave. If either (or both) of you really believe he killed someone, you ought to be able to find out more. Where is "up north"? When would he have been there?
You might go talk to Mike Wenk, the D.A. Tell him that this guy said he killed someone and you have reason to believe it is true. Find out what information he thinks that the police would need to file charges.
The reality is that she needs to leave him. You both need to stop screwing around with half-baked ideas of locking him up for a murder he most likely never committed, and come up with a plan to get her the hell out of there. You need to go talk to someone at a women?s shelter (there is one in San Marcos) and find out what she needs to do to get away from him safely.