This is tricky. I think you definitely need to complement him for going. Can you mention some mundane custody issue and then say oh and D is so happy you did the prom thing. It was so great of you....
I think the key is to admit to the D before you pat him on his back.
---- M 39 H 35 D5,D4 M 4 T 9 ILYBNILWY 5/18/11 Left 7/11/11 Divorced 12/1/13