AND as you're not dating the girl in question, it's really none of your business. You're not (yet) an important part of that particular story, you barely talk to her. What he's doing or saying has no bearing on anything going on in your life. I'd be willing to bet that he doesn't know you exist. And so far, he's got no reason to care.
And in all of those similar stories, it's none of your business. It's the GIRL'S place to take care of the ex, or not. If you had some ex and there was some drama there, some new girl that's barely speaking to you (much less anything else) wouldn't be overly welcomed into making decisions and getting hostile/aggressive with the ex, would she? Especially if you weren't sure you had any particular interest in the new girl, you barely talked, and you hadn't fully worked out the deal with your ex yet? That's essentially what it looks like from the outside.
You've got all this going on in your head, and you're way over-invested compared to the girl. There's a whole story in your head for what's going on, but it's only in your head. Until you get involved with the girl, none of your business. And even then, not really your business unless the guy physically attacks you. It's up to the girl to tell him to get lost, or tolerate him, or ditch you to be with him, but being aggressive and possessive doesn't help your position.