My ex and I are best friends.We speak to each other almost daily and when I'm in bloem we hang out together or just sit talk about shit in our lives. It depends, I think, on what kind of relationship you had. If it was all physical, then you can't be friends, but if you were together for long and were able to talk to each other and share thoughts, its possible.
It is tempting sometimes, but you have to consider that you may screw things up if you do any more.