Resist the urge

Sometimes no matter how strongly you dislike or distrust a person, it is better to keep your mouth shut and let the person involved with them make their own mistakes. It is hard to do, but the person will not thank you for your opinion, they will attack you and you will end up looking like the bad one. They will twist your words and make out you are trying to control them, just because you want to help them. This is also true of people who come to you begging for advice about someone. They already know themselves the person they are involved with is bad news, but they want someone to lie to them and say how wonderful they are. They can not handle the truth, they don't want to accept it and so you become the victim of all their pent up anger and insecurity that they hide. They would rather fool themselves and take the long route to truth, than be told by someone else.