Oh, a whistle is absolutely the tool for the job, no question.

Relevant quote from the article:

“He is wearing an ankle monitor, so he is not allowed to come within a certain distance of where I am, and, of course, my work and whatnot,” she said of the man.

“But even then, you get a heads up of a couple of minutes, the police can’t get there in time. If this person wanted to be violent, they could do so within that period of time.”