Dude if you think even half the people in the world have the strength to do that...
Ultimately, then, it's on you. No one is ever going to be as strong an advocate for your own life as you will be. If you can't help yourself, there's not much that can be done.
You can't control other people's selfish actions. You can control your responses to those actions. I'm sure this girl has had a litany of friends and acquaintances tell her what is really obvious to everyone watching a few minutes of video. If she doesn't make a break it's her fault at that point. Definition of insanity and all that.
Also, I get the point of 'she can do better', but focusing on the fact that she's attractive seems like it's undercutting her. No, ugly people don't deserve to be cheated on any more than her, and it doesn't matter if you're less attractive, you should be getting out of those relationships anyhow.
I'm not trying to brag but it's never been hard for me to meet women, and I have no thoughts on cheating on my wife at all. 34 years old. Sounds more like you are projecting.
After all the Gamergate alt accounts and the "Be sensitive about the Nazis" stuff recently, I've given up on expecting juniors to have any sort of worthwhile insight.