Guys, I need some advice.

So about a week ago now, I (16m) started dating a friend of mine (16f). She told me she liked me and I liked her too, and we really hit it off after that. However, today we almost broke up, no, we didn't get into a fight, but it's because us being together is causing some issues. I told her not to listen to anyone who talks down on us, but she just doesn't want me to be hurt. I ended up convincing her to stay with me, and we've been together since. But what kind of problems, you might ask? Well, it's mainly because a friend of her's is jealous. See he's known her for over a year before I met her, and he has an obvious, obsessive crush on her. He's manipulative, and tries to drift me and her apart, and make her choose between him and her friends, including me. She's rejected him several times but he just keeps asking her to break up with me and date him. I was actually hanging out with her today and he was there. Out of nowhere he started making outlandish claims like "I'm hiding who I really am from her and I'm actually a horrible person" and even threatened me. I've tried to be nice and make peace with him several times, but he's not having it. Me and her came to an agreement that I won't be around him anymore for both of our sakes, but now I can't spend as much time with her because he's always around her. She still cares for him, and I don't want to break her heart, (too many other guys have and I care for her too much), but she's in a toxic relationship with this person, and I want to help her out of it. Or at least get her to try and talk to him.

How would I, in the nicest way possible, explain to her that she's in a toxic relationship, and try to help free her from it? I really care for her, and I don't wanna seem like I'm trying to "take him away from her" like he keeps saying I am, but he's an obsessive and manipulative "friend" who I personally think my girlfriend would be better off without.