You can't save her, and that's okay.
It sounds cliche, but it's a cliche for a reason. We all have that one friend or family member who can't seem to let go of a lover who is just not good to them. For some reason, she keeps taking him back even after he cheats on her, lies to her, stabs her in the back, damages her self-worth and pisses off all of her friends and family.
You're her friend. You love her and you've despised his existence ever since the first time he made her cry. You've tried reasoning with her...
"Girl, if he cheated on you he'll do it again..."
...but she never listens. It's like she's only a shell of the friend you once knew and loved. Like ever since he broke her heart and she took him back, she's been sleep-walking and unable to wake up. It makes sense considering the number of times you've probably wanted to slap her since this whole thing started.
Not only has he infiltrated her mind, he's reprogrammed her thoughts and made her think that she won't be happy without him. It's been proven that love (and heartbreak) can be more powerful than cocaine. This phenomenon in our brains can cause us to let go of all judgement and can lead to self-destructive behavior.
Heartbreak is more powerful than cocaine
Abusers use certain tactics, whether they know it or not, to maintain control over their victim (yes, this is an abusive/unhealthy relationship). While your friend will have to accept responsibility for her part in it later on (assuming she wakes up and doesn't waste the rest of her life with this guy) her current state of mind is basically tunnel vision. Much like a drug addict, the high of the "honeymoon phase" is the only thing keeping her going right now.
So you can keep having interventions, telling her she deserves better and waiting for the day that she finally wakes up. Or you can show her some tough love.
People don't change or wake up unless their world is completely shaken. Sometimes it takes great suffering for change to come. Maybe this guy would change if he got a taste of his own medicine and your friend decided to leave him and break his heart for once.
But considering her current state of mind, it's more likely that you will understand this before she does. And that's why it's not a bad idea for YOU to take a break.
Hear me out...
When someone else's unhealthy relationship is negatively affecting you, it's time to insert some space for your wellbeing. It's not easy, but it sends a stronger message than any of your interventions or pep-talks did.
The thing about tough-love, is that you have to stick to it. You can't say you need space, but only take a few days to yourself and revert back to the way things were. The only message that kind of behavior sends to your friend is that you aren't serious about your intentions and gives her the green light to continue on her path of self-destruction while still getting to have you in her life.
I'm not saying you should cut your friend off completely, but after countless talks and efforts to try to help her, it might be time to accept the fact that your efforts are futile.
Unfortunately, you can't help someone that doesn't want help and sugarcoats the problem. The only thing you can control are your actions, and the only power you have right now is over your presence in her life. As sad as it may be, it might be a wake-up call for her.
Before you start distancing yourself, it's always a good idea to make it clear that your friendship isn't over, it's just on a hiatus, and that when she decides to wake up and leave him, you'll be there for her no questions asked. But for now, you can't support her self-destructive ways.
What to do when she comes crawling back...
Because she will... Just hope that it's not 20 years and 2 kids later.
She may never admit to you that she was wrong, but if she does, don't say "I told you so." Just be glad you have your friend back in one piece, even if her heart isn't. True friends help each other pick up the pieces and in the end, she'll love you even more for it.