The one constant is the sad clinging to the belief that your s/o is the exception.
She did it. She is that kind of person. Far more likely than her just "fessin up" was the fear that you were going to hear about in anyway.
Apply reason. No one just "sends" nude pics to people they know. It is done for a reason. There aren't that many so it isn't like you have to sort through a few dozen to find the right one. Somehow I suspect if he was a short,fat, ugly 300lb guy she would have found it in herself to say no when he asked.
Just a guess