How about this? You go meet with the two of them together (without your daughter) and tell them both what D saw. Sure, it’ll drop a bomb, but it will likely put the focus off of D and onto the revelation between the two of them. It also makes it a little harder for ex to gaslight her with both of you there.Then let them know you’ll be keeping D with you until they figure their sh!t out.
Or, if you prefer to be out of the line of fire, you could just tell his wife first and let her handle it how she wants. She might want to get some ducks in a row with finances etc before letting him know what she knows.
