There's really not much you can do, at this point. You've told her how you feel, you've expressed your concern, and anything else probably isn't going to be taken well.
It's pretty straightforward: she doesn't want to be the fool. No one does. So she's not going to listen to you because doing so would mean, well, embarrassment. Denial is a powerful force.
I understand the desire to protect your friends, because dating a cheater sucks in so, so many ways. You want the best for the people you love. But realistically speaking, this is a lesson she's going to have to learn for herself, like it or not. Just be there for her when it comes crashing down.
Sorry that you know the truth and she won't see it right now. Eventually she will (or maybe she won't). Either way, she is going to resent you if you keep shining the mirror in her face. She's not ready to leave him. There are deeper issues there.
We leave the state of denial in our own time. After that, she might thank you for being a good friend. Real friends tell us the truth, and true friends will appreciate it...eventually.
Good luck.