Friend who Chooses Poor Relationships?

I have a friend of 6 years who used to be married, but is now divorced. While it is his choice who he dates, I have a problem with his current choice. His current flame is still married to her husband, though they have been separated for 1 year. She left the husband and has been dating a co-worker of mine for the past year and is still dating him, and has no plans to get a divorce or break up with the boyfriend--she 'doesn't want to hurt their feelings'. So, now, my friend wants to bring this chick around who I know is dating someone I work with. I can't lie to my coworker's face when I see him at work. I can't knowingly be a part of her cheating on him. We are best friends, so we spend every weekend together. Is it wrong of me to request that he not bring her around me until she is broken up? What about the marriage, as I don't personally know that guy, but still find it disturbing that she wouldn't end one relationship before starting 2 others. Thanks!