Not sure if it was really an accident or not. I was staying at a friend’s place several years ago, and one night he decided to show me some vacation pictures a few years earlier with an ex-girlfriend who was also a friend of mine (through him but we kept up the friendship after they were broken up).
Suddenly there was a nude photo of her from the hotel room, she was getting ready for a bath I think.
At first he apologised, but after seeing my positive reaction he showed me some more pictures of her in various states of undress.

After that he showed me his collection of pictures of other exes, and some pictures from a party that ended in skinny dipping, I didn’t know any of the girls but there were plenty of nice ones.

It turned me on immensely, and later I downloaded the entire collection to a USB. I still have them and look at them quite often. He told me in passing a long time after this that his current wife made him delete them all. Don’t know if he kept a copy secretly or not, but if not I’m the only one in possession of those pictures now. He doesn’t know I have them and never will.
And I’ll never show them to anyone else, they’re spirited away on my hard drive.

What struck me as extraordinary about your story, wasn't seeing nude photos we shouldn't (a common fantasy + hijab adds to the fantasy). But that you let slip that she is his gf not wife - "living in sin" for 10+ years, yet is a "very modest and religious" hijabed muslim woman.

Yep, I'm the geeky friend that everyone needs. Buddy was getting a new phone, I warned him that the phone stores say they will transfer everything, photos, contacts etc., to new phone, but more often than not, they screw up and you lose stuff. I offer to show hint how to back up his phone to his PC, but he said he'd rather I do it. So I backed it up to my PC and away he went to get new phone.

Sure enough, only the last 30 days was replaced, so he gave me his new phone, I told him it would only take a couple of hours at most. After I was done I gave him his new phone, we started flipping through a few pics and the was his cute wife topless with him on a beach. He cracked up, so I kept going, no sex acts, but several more of her fully nude. He swore me to secrecy.

I guess you have a point. She sure plays it off as if she’s extremely modest. Hopefully I get to see more of her sinfulness.