Tell her she commuting adultery, and ask her how will she feel her husband was cheating on her, and if she say I don't care then go tell him she's cheating on you because eventually she's going to end up getting pregnant probably and he will find out some how
tell him! if you were in her finances shoes would you like it if they kept things from you especially something so big.. Im sure you won't . and your doing both your best friend and her fiance a favor. obviously your friend isn't ready to settle down and get married.
Talk to her about it. Sometimes people cheat because they are unhappy with the relationship they are in. Since you are her best friend it will probobly be easier for her to talk to you than anyone else. The conversation won't be easy though. I hope this helps!!!
Tell him but make sure you have proof. Then walk away and don't look back. She's not much of a friend anyway and she'll hate you for telling him. He'll be hurt of course. He may hate you or be glad you told him. Best thing let them deal with it. Good luck.
If you have a friendship with him you have to tell him. If you don't I think you should still tell him. Unless you have no morals then don't tell him. If your girlfriend is that type of woman and you agree with what she is doing then don't say anything. If you don't agree then tell and maybe you should not have a friend that would put you in that kind of a situation that's kind of selfish and inconsiderate of her. Good luck.
It's hard to believe he doesn't know something about this right now. But anyway ask yourself would you want someone like this marrying into your family? Bringing all this heartache and drama with her. She is not ready to be married, but she has got this guy on the hook and she is not willing to let him go. TELL HIM, TELL HIM, TELL HIM! If this were your son or brother you wouldn't hesitate. Save this poor guy from himself!