If the nice lady does not find someone else to fixate on you may have to get the services of either a lawyer or the police. Do not procastinate too much.

The next time if it occurs will be too much in my mind. At that point thinking she will still go away is too risky.

People with bad intent can usually find a way honest or otherwise to damage a person. Remember you are in all likelyhood not the first. She obviouisly has some issues. Always beware any evidence of a mentally unbalanced person. Keep a copy of the last email as well.

You have a real winner on your hands I suspect. I also suspect you already know that her performance is already over the edge. Whatever you do at present do not respond directly to her yourself. This behaviour could escallate as a result.

You temporarily are one step ahead of her if she does not know you know about her letter to the boss. There are always going to be a few people out there with major issues in society.