Could our best friends be our worst enemies?

Don't get me wrong I am all about girl power and I totally believe in the strength of friendships but after reading an article in Marie Claire about best friends it seriously got me thinking. Could our best friends be our worst enemies?
The article is mainly about always agreeing with our friends. For example one of your best girls breaks up with her boyfriend and of course when she comes to you for advice you simply explain "you were too good for him!" and "yay he was an ass!" But what if you didn't really think that and you were just agreeing with your friend who probably said the same thing out of anger by being the dumper or the dumpee.
I see it everyday with my friends and I. They come to me asking if they made the right decision and usually I say "of course you did and that you are better off without him" but sometimes I am thinking in my head what a big mistake that was and how I really did think they were good for each other. I will usually sneak in something about how I think they should work things out... etc.

But why as women do we always feel that we have to agree with each other? Do we feel as if we are breaking some code of friendship if we don't offer sentiments of happiness when someone breaks up with their boyfriend? I guess I would rather have friends who disagreed with me than always agreed with me.