I think each sitch is different. Was she actively cheating with your H while you two were married? If so, I would not interact.
In my case, I met my lady while they were in the final stages of the divorce process. I have attended daddy daughter dances with her X, their daughter who lived with me full time and my daughter. We sit together and we are civil.
My X wife ignores me and my lady. Its only been 10 years...
"What is best for my kids is best for me" Amor Fati Link to quotes: https://www.divorcebusting.com/forums/ubbthreads.php?ubb=showflat&Number=2879712