You sound like a fair weather friend. And really judgy to boot.
In my experience, it takes a long time for a married woman to jump ship. You have no idea what has led her down this path. She must have been extremely unhappy in her marriage to do this.
I discovered who my fair weather friends were, when I left my ExH. My friends of 20 years, didn't stand by me. They didn't even ask why I was leaving. If they had, I could have told them that my H had cheated on me with 10 women, and occasionally beat me up. But no, they didn't ask, no doubt talked about me with pursed lips. Ironically, the one friend who was an absolute rock, had only known me for about 5 years. She was amazing.
I also found that some people (my parents) wanted to keep me in the marriage box. Anything else was too uncomfortable. But you can't confine an unhappy person to a marriage that they don't want to be in any more, just because she thought 20 years ago that was what she wanted. Her H may have changed, she may have changed, as people do. She is allowed to leave him and start a different life if she wants! I certainly did just that and I am very happy with my 2nd DH.
My advice to you would be to support your friend. She needs you now, more than ever. If you can't do that, she will never really see you as a friend when this is over.