So...I gave them the diaper cake and it was...awkward.
They had no idea I knew and were none too pleased (but not all that surprised) when they found out the info came from my mother. His girlfriend actually seemed grateful. She told me she was planning to use it at her baby shower. The ex was...stunned - and a bit confused - like they were going to find a deadly snake in the middle of all those diapers.
All in all, it was an interesting experience. It actually (and surprisingly) gave me some closure- both the crafting and the gifting. And, I really appreciate all of the support I've received from you all!
Oh, the best part: their baby is due on January 1st- my birthday.
Thanks, again, Craftsters. You all rock!