I have said as much to them. They have told me numerous times that this isn't about me, but how they will not accept the adultery-partner into their family. Knowing how his parents feel, I don't understand why ex would continue to insist that she go to these things. It is their battle of wills, at this point. His parents are not willing to gloss over how their relationship started; I think they blame her for the affair much more than him (as I did for quite a long time).
Before they left for the weekend, my son asked me (and his grandmother when I wasn't around) why his father felt compelled to bring his gf and all the other kids. I answered that I don't know...
