If I were you, I would tell her in no uncertain terms to not go there
I wouldn't care if she decided she could no longer be my friend, or thought me judgmental
After that, I would say if she did go ahead, she did not have my support and will not be prepared to discuss it further. I would certainly not be a sounding board while it was happening, although if she was a friend I otherwise valued I would consider being there for her when it all goes tits-up (it will) if she didn't otherwise make a habit of being utterly fucking stupid
I have a close friend who has a terrible track record for relationships and falling for guys who aren't interested in a relationship. She has recently started a new job and is very fond of one of her new colleagues. He is married and has an 18 year old dc. He has told her quite regularly that he lives an "alternative lifestyle" which my friend believes means he has an open relationship as he is quite touchy with people and open about his sex life. They have had a drunken one night stand and he is offering / requesting more, although my friend is unsure about whether he is looking for a "no-strings relationship" or an actual relationship. She has consulted me for advice about what to do, but in a way that it is clear she is asking for me to help her think it is okay. I want to advise her not to continue with him even if it turns out that he does have a truly open relationship with his wife because they have to work together in a very public facing role for at least the next year. However I am finding it very hard. I am tempted to just say that I don't want to talk about it and move to another topic. I wouldn't know how to phrase my concerns in a way which does not sound judgemental or like I am being old / snobby because I have a dh and dc. I would appreciate any advice!