My 1st ex-husband married his best friends wife, they have a son who is 3 months older than our son together...she was supposed to have been my friend too. My son calls her mom. I realize that she is trash, a habitual liar, & in general an idiot...but I'd still rather deal with her than my ex.
My 2nd ex doesn't seem to be able to keep a steady girlfriend for long. When I met the woman he was cheating on me with, it was in a parking lot where he was taking the kids back up to MI for one month. After I told the ex what a ugly skank she was (out of kids earshot of course)...I only said one thing to her, "I don't know what you are all about but don't let him hurt my kids."

Yes, I say pretend if you must but try to keep at least a civil relationship.