JMO, but I don't believe ex's can be friends. People who love and respect one another ARE friends. In fact, I think married couples who are each other's best friend have pretty darn good marriages. When people break up or get divorced it's because the friendship side of the relationship went down the toilet. Nope. People who divorce and remain "friends" may have a working relationship for the sake of the kids or to keep up their image of having a "civilized" divorce, but that's as far as it goes, in my opinion.
On the other hand, I don't think one needs to bad-mouth the other or hate them. Anger is a natural process of grieving the loss, but I don't think anyone needs to get stuck in that phase. When my exAH and I broke up, we had contact a few times, then he moved some girl in with him a month after he met her and all I could think was, "poor girl!" Anyway, I cut my losses and moved on. Didn't see the need to be a friend or an enemy. I was just relieved to have him out of my life!