Sometimes in life you need for a limited period of time to accept something to gain something. Dealing with a WH is like playing chess, you need to know when and where to move your pieces. Acting/thinking versus reacting...
I am fully to set up and enforcing boundaries, but right now, their situation is "really bad", he moved out already, so the main goal is to deflate some of the anger he has in regard of his relationship with TO. There is not way to "re-attrack someone" when anger is at its peak. This is an emergency crisis, so the more he is "willing" to come over and the more positive is that experience for him, its always a step forward in favor of the family bond. That's why DB is about detaching and "acting as a friend" versus nagging, begging and being overly critical. The goal is to reverse the scale from negative to neutral then to positive.
Also, the more he will come over, the more "unhappy" Ow might become. An unhappy OW is not full fantasy anymore.
Me 52+ WH 57+ Married 20 + Piecing since 2016 (ready to give up...)