In my opinion, it is possible to be friends again to a certain degree. Long story short, I am going thru a similar situation. I get to be friends with my cheating former spouse for the sake of our kid... She left me and our child and moved in with him.
At the begging, It was so hard to even talk to her. I even begged her to come back once (i know it is pathetic). However, after a few months of therapy, soul searching, and a lot of willpower. I managed to establish a peaceful arrangement with her. It is still far from perfect. She has asked me at least once if I'll be willing to try again! Now, I try to avoid any conversations about our "good times" together when she tries to bring it up, or anything that has to do with "us" It is an uphill battle... Just try to remind yourself that this is your parental partner and NOTHING else and you have be friends with him/her because of the better good. Also, never forget that you are an amazing human being and that you deserved better. Hope this helps.