Is it still infidelity if they say...

[newmama]

I am so sorry. Well the infidelity depends...did he start with OW when you were still married? If so, then yes it counts. Of course when you are still legally married and not legally separated than a relationship with someone else technically counts.
You can still use the divorcebusting techniques though...especially since it sounds like he thinks of you as a best friend and doesn't want to let it go. If my H divorces me, our friendship is over. We will remain business partners as parents. But that is my boundary...