Ugh, I understand! I agree, you are really in between a rock and a hard place. Let me tell you though, HR is not your friend. HR is there to protect the company, not you, not her. The same would go for your boss, the owner. He isn't "on your side" he is on his side, and if you and this co-worker become too much of a problem, you will both be seen out the door. I would do everything you possibly could to deal with this without bringing him into it any longer, since he has already shown his unwillingness to be proactive. I am speaking from experience here.

I moved into a new apartment when I got a new, better paying job, and it turned out that I moved upstairs from (my now husband) the exh of a coworker. I supported the SVP/Legal Counsel at the office. I did not work directly with the coworker, had no reason to have any dealings with her at all. But when she found out I was dating her exh, she went bat **** crazy. She harassed me at work, rifled through my desk, threatened me, harassed her exh while she was a work. I went to HR and to my manager, filed 3 formal complaints, and nothing happened. Then just before Christmas, she was fired, for poor work performance, and then after Christmas, I was told that I had a choice of either being laid off, with a huge severance package, IF I signed a confidentiality agreement and agreement not to SUE them. If I didn't sign it, they would fire me for cause, they would find one. Knowing the fight that would ensue because I supported the legal counsel and had seen them do similar to other employees, I knew I had very little choice in the matter. It was a matter that they didn't want to deal with, they just wanted to protect the company and that was it.

I sure do not want to see anything like this happen to you!

Can you respond to one of her emails criticizing you with a request that she not have any involvement with your work considering she doesn't have anything to do with it. Request that all issues with your work be directed to your manager, or something to that affect? Keep ALL emails, print and take them home or forward them to your personal email. I am sorry you are dealing with this, I know how it can take over your life.