Another gal really hits close to home anyway

So I'm really curious about how someone who doesn't respect boundaries will make a relationship work. Like there was this girl-let's call her the boundary breaker-she'd hang out with my friend all the time, even though she knew he had a girlfriend at the time. Honestly, it's like they were besties or something, and I kept telling him to back off because I wasn't okay with it. But then after our breakup, witihn two days, boom-they were together again. So here's what I'm wondering: do people who act this way eer stick around in a relationship? I mean, is it even possible for someone like that to last long-term?