Here is another scenario for you. Your best friend has been dating a man that you and all your friends despise. You've heard all kinds of nasty rumors about his past, that he's been cheating on your friend, etc. They have only been together for about six months and you've never let on that you didn't like him in hopes they wouldn't make it, but low and behold, you found out that he's planning on popping the question.
So now what? Do you trust your gut instincts about this man and tell your friend that you think she's making a mistake with this man, even though it could taint your friendship? Or do you stay mum, not get involved, and let her live her own life?