Ok, ok, trying to harass her.
No, not okay. He's not trying to harass her, he's trying to befriend her.
Please don't push him away.
You people are doing this wrong. He's not going to stop just because you tell him to. Because he's obsessed. People don't just go "oh, okay. That makes sense." And stop obsessing over something just because you advise them to stop obsessing over something. This is not a rational thing. He might be able to pursue the obsession in a rational manner, but the obsession will remain irrational as that is the nature of the beast.
So I think the best we can do is try and minimise the damage to both parties. Killing the obsession might be possible, but I believe it would centre on convincing him honestly that there is absolutely zero possibility of it ever coming to fruition, and there being no possible path to a possibility that results in a positive outcome. To that end I'd be interested in learning about what he intends to communicate and the history between the pair. If the history is too hostile, then there's no way this will work. If it's not so bad, and it is at least several years since they've last interacted with one another, there might be a possibility. And by possibility I mean a really slim one based on what I've read so far. I think he'd have to be sure any and all the problems that had happened between them in the past would no longer be an issue and that his presence in her life would be a positive thing for her for there to be any chance at all.
This girl I don't think is good for him. She doesn't seem to have anything to base a relationship on in terms of having things in common, or even be especially pretty, correct me if I'm wrong about any of this Aardwolf. Yet somehow she is powerfully attractive all the same, and it seems to be at least from what I've read pretty much nothing but that attraction that is drawing him to her. I wonder, what will he do if he gets to talk to her? Does he even know what he would do with her? Does he want to do anything other than bask in her presence? The important thing for him seems to simply be for her to be around because of how it makes him feel. Perhaps something akin to an addiction. This in itself is very dangerous I think, for someone to have that kind of power over you. It could be exploited to control or hurt you...