But it’s important to see it for what it is, rather than thinking of the ex as the only one who messed up. If he’s kind and respectful about the others, that’s a good sign.
I cannot understand why out of all the places to work, she chooses her ex-boyfriend’s old job, and where his current girlfriend works.
But it is another when you have to see it at your job every day.
Now I know she is there, I feel I need to make a presence of some kind.
Or sure, it’s also possible that it’s some kind of weird attempt to mess with you and/or your boyfriend — but until you actually see evidence that she’s doing that, you should proceed as if it’s not.
To be clear, if she had been threatening to you in the past, that’s something you should share with your employer now. If she trying to make some sort of point to you, the best possible point that you could make in return is to just be normal.