Respectfully, I would recommend doing whatever is best for DD, putting aside whether he wants to be with STBXW or not, and which, if I know him, is having both parents present at her birthday party, as long as he's up to it (which I know he is, despite having frequent daydreams of punching her) .
When the men on the chessboard Get up and tell you where to go; And you've just had some kind of mushroom And your mind is moving slow; Go ask Alice... I think she'll know.