Breathe! You want to be as calm as possible when dealing w/him on this issue. I would either call or text him and set up a meeting to speak to him privately about this issue and advise him of what your daughter saw on his iPad. I would ask him to please change the password and not give it to the kids again if he's going to be storing nude photos on it.
BTW, I would be furious too....but you will get your point across more effectively if you stay calm and speak in a level tone. Otherwise, he'll put the blame on your child for using his electronic equipment w/o his knowledge. They love to blame others for their stupidity.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
