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This method likely would have worked by itself. The method that worked sooner wasn't just "another photo" found online of her friends as grecy mentioned. It was a photo we had taken ourselves, and it didn't have anyone else in it. But I'm not ready to tell Justin more details about it.

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This strikes me as odd, albeit a clue. You had a photo of a girl you'd presumably never met. A photo you'd taken, that you didn't know was her at first.

They went to the same event, took some photos. And in checking their photos, see that the same girl was in their photos too. Then they saw who she was with, and were able to locate a friend of hers. From locating a friend of hers from this second photo, they located her.

"didn't have anyone else in it" - seems no one else was in the photo with her.

Perhaps she was doing a presentation, so was alone at the front of the room. The photographer, being in the audience, would obviously know which presentation it was and from there its a quick look-up on the list of presenters.

Maybe her phone's contact list, call history, Facebook, Instagram, etc. or her business/credit card were visible in that picture?

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