How do you think this will end?

In July, our group of friends went to Vegas for a week to celebrate my best friend’s birthday. Because it was her birthday, she brought her two sisters with her, one is married and one is single. One of our guy friends was flirting with my best friend’s married sister at the beginning of our vacation and then they became very touchy and close toward the end of the week. We didn’t think much of it because he is always the flirting type, but then after we left Vegas they decided to kick it off. My best friend told me that her married sister is seeing our guy friend. We are not sure if they catch feelings while together in Vegas, but my best friend told me that her married sister slept with our friend in Vegas. She should have separated from her husband first, before she chose to spread her legs for another man. I don’t see a good ending to this at all. Why can’t she refuse him when he made advances? Maybe it was a mistake?