Couples Who Should Break-Up

Do you have friends who are in a relationship but they are completely wrong for each other? They are both good people and you are friends with them as a couple, or do not have a preference for one member of the couple over the other. They don't necessarily hurt each other but it is obvious to you that they are just not compatable. What do you do? Do you try to tell them? Do you think they already know, especially because it is so obvious to you? I wonder if I've ever been one of those people in the relationship and no one told me.