I too would have difficulty remaining friends. This pains me to say, because I considered this woman to be my very best friend whom I contacted several times a day. I miss the inside jokes we would share and her laughter. Since we have a daughter, we will have to be civil (or that is my goal anyway).
Back to the friends topic. I have essentially had four serious girlfriends prior to getting married (I did not marry until 34), and although if I saw any of them I am certain we would share a hug and be able to enjoy each other's company, I have not remained 'friends' with any of them.
I used to exchange holiday cards with my first GF (16-21), but I stopped sending one...not sure why, but I felt odd sending a card once a year and I did not want her childeren getting older and asking who was this guy from another state sending a card. However, perhaps it would have been good for her to show her children that x's can actually get along.
Me: 48 Ex-W: 45 M: Nov '96, together since Oct 93 Bomb: on 10-yr anni - Nov '06 OM Separated: mid-Feb '07 Divorced mid-July '08 One daughter - 28 XW living w/OM