Just show love :)
I don't have to deal with it now, but when married to my ex I had a few dealings with his ex wife who was also the mother of his children AND high school sweetheart. My advice is nog popular or easy. But remember that at some point they cared for eachother. Whatever circumstances came about to end that happened for a reason and has nothing to do with you. Be the women your man has chosen and he'll remember why he's not with her. Be supportive but firm. The last thing a man wants to come home to is a woman who thinks about his past.