Stalker. Any advice? in Hello 👋🏽

A girl from my boyfriends past has come back and she’s driving us crazy!
She was never his girlfriend, it was more of a friends with benefits type of thing. I guess somewhere in between of all that, she got some really deep feelings for him, feelings that he never had for her. When me and him met, he told her that he wasn’t interested in continuing their ‘fling’ and he wanted to focus on someone else (aka me). She never stopped trying to talk to him even though he would never respond or give the time of day. She left for a couple of months and came back this week. She messaged him when she’s be back, instead of a response he blocked her. The day she came back she went to his job on purpose to try and see him,’he never showed his face to her. That same day, when he got out of work, she was driving around his house (she lives nowhere near him or his job). She’s been doing that since she got back. He told me that even before I came into the picture, he tried to get her to leave him alone and she just did the same thing she’s doing now. She would even go as far as waiting for him by his car when he got out of work.
I honestly don’t understand what her problem is. If he’s not interested, then he’s not interested.
I do understand that the way he ‘broke things off’ with her wasn’t the best, but if someone isn’t interested in you the best thing to do is to leave them alone. Not Stalk Them! I feel like she has no limits and if this continues I’m going to see if he could get a restraining order against her because this is honestly ridiculous!
I just want to know why she’s doing what she’s doing!