I Can't Save Her This Time

On another note, entirely unrelated to my health:: I found out two days ago that a friend I love very dearly has gotten into a relationship with a married man. This guy is 18 years older than her and his wife has cancer. It breaks my heart to see her do this to herself. I have this pit in my stomach telling me that when it ends, and it will end, it will crush her so thoroughly. Yet she refuses to listen to my warnings. I tried to tell her that this is wrong. I tried to tell her that it will very likely end badly for her. I can see it from here, that he's using her. He's got her convinced that his wife is a crazy shrew. That they haven't been "acting as man and wife for some time". Yet he's made no plans to divorce his wife. He's leaving it up to his wife if their marriage is to end. So where does that leave my friend? Squarely in a bad position.A position she is more than intent on keeping herself in. She's justified this to herself and is unwilling to see reason.
I feel awful for it, but I've pretty well deleted her from my life. I have spent that last several years as her friend, doing everything in my power to save her from her own decisions. Trying to get her through the consequences of her actions. Talking her through all the rough patches. But now she's in her 20s. She's an adult. And she's very clearly no longer wanting to listen to me. So she's on her own, this time. I can't stand by and watch her do this.
For me, marriage is sacred. It doesn't matter if the relationship hasn't been great for a while, you are still bound by law and by God. No matter how bad the relationship is, you're still married. Nothing erases that fact. If you're done with your relationship, then tell your partner and get a divorce. You don't cheat. Ever. There is never an excuse for it. Just cowardice and selfishness. Feeling as I do, I can't and won't watch her willingly be the other woman. It disgusts and saddens me to know that she wants this for her life. I hope and pray she comes to her senses before it's too late.