Trust an attention-centered girlfriend ALONE with my fiance?

My fiance and I have been hanging out with this fun married couple who have been married over 6 years. Everything was fine until last week. She told me that she cheated on her husband with another married man who has 2 young children. Not only that, but in her spare time, she has been staying out all hours of the night with her male coworkers, (to the dismay of her husband) and that she “absolutely loves the attention” that she’s been getting from these men. Recently, she’s been asking my fiance out ALONE to lunch and it totally scares me. (He said ‘no’) If it were any of my other friends, I would not care. But due to the fact that her husband doesn’t know she cheated and she freely acts so permiscuously with all of these other men, I’m afraid that she has no moral boundaries. I’ve been friends with this girl for 5 years and this attention-centered behavior only started about 6 months ago. Do I end this friendship? What can I do?