Re: OK.. I need some serious advice!
In my first marriage we both had multiple affairs.
ExH had many affairs because that's just what he does. The man is broken. He cheated on me. According to the grapevine, he cheated on all of the women he dated after the divorce. He remarried about a decade ago and, according to his mom and my daughters, he cheats on her, too.
I cheated because I made the foolish mistake of marrying someone solely because we were having casual sex and I got pregnant. I was miserable with the man, thought the best thing was to stay for the kid(s), and had lovers to meet my physical, intellectual, and emotional needs. I met and fell in love with my last OM, left the sham marriage, married him, and have been faithful for 16 years.
Clearly, not everyone who has cheated in a relationship is going to cheat in other relationships. And, equally clear, some people are just cheaters.
That said, if this woman was truly unhappy in her marriage, she'd leave. She has no excuse to stay. They don't have kids and she has a job, so she can support herself. If she's staying, it`s because she wants to.
So, tell her she has to either end her marriage or you have to end the "friendship". If she is legitimately unhappy, she'll leave. If not, she's full of sh!t and staying married because she wants to be with her husband.
It took me less than a month to end my first marriage after getting involved with my then AP and now husband. That was with me having 2 young children and no job. This woman has no kids and a job, so she could literally go home today and tell her husband it's over and then be in a lawyers office by tomorrow. So, don't accept excuses. Give her maybe a week or two to end her marriage or you cease any and all contact with her.