Ciara, when you say "break it" we assume you mean break up, which implies that the two were originally boyfriend/girlfriend, and are trying to be something else (which usually still involves sex).
In that case, I'd say pure friendship is possible only after the two stop having sex, and both have had a real relationship with someone else since separating. Even then, I think it requires that the break up was on good terms.
In the case of two friends who weren't previously together, I think friendship is possible as long as they didn't only become friends because one person wanted more and couldn't get it. If I'm in a happy relationship, it's easy to be friends with girls because
I don't want to bang them all the time. Even if I become single again, I already think of the girl as a friend. But, if I'm single, there's no reason that I wouldn't want to hook up with a pretty girl that has a personality worth being friends with, so I'm
probably just friends with her because she won't let me bang her. Like Jon Gu said. :-p