Would you consider this a threat and if so what can I do about it? (long)?

First --do not delete the letter-- save it and print it. For some reason she's hearing things from someone ( maybe her son). I would block her and private your page so no one that they know can see it. No fuel for the fire. If she contacts you again call the police. Don't in any way contact him, her or their friends, family for any reason. People like that are looking for trouble. Don't even talk about them. There's to many drama queens out there that are mental cases.