It just depends on the people involved. I can say beyond a shadow of a doubt my ex and I will never be friends. We didn't get along when we were married, we don't get along now, and we never will. If you and your ex are capable of being civil that may eventually lead to a friendship. It probably helps if both you actually want to be friends. Plenty of people manage to do it, but it's really up to you and her and the circumstances.