I really wanted to tell d-11 : "You want to spend the night at the house of the man who wrecked your family and stole your mother?"
Should I have said SOMETHING negative?
No, as hard as it will be you don't need to say things like that to her. You won't believe this but I've actually seen cases like this backfire. I think it is b/c one parent (say you for instance) is so hurt and feels offended if the child goes to be with the other parent who did all the damage. It puts too much pressure on the child and then the child feels they have to make a choice in which parent he/she will be with b/c they can't handle the tug of war.
She is still the mother and no matter what she does her child will love her and wantto be with her. The more you say against the mother, the more chances you may be taking in pushing the child away from you.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!