During the Brotherhood era either shortly before Prompto becomes friends with Noctis(or shortly after as long as he hasn't met Ignis yet) Prompto got an opportunity to assign a famous or semi-famous photographer. He thinks that's it's just going to be adjusting lights, or dealing with props, but when he gets there it turns out the model "canceled" and is talked into taking the model's place. As the shoot goes on it becomes more and more explicit and erotic until it's full-blown porn. Prompto walks away feeling vaguely uncomfortable but trying to convince himself he's totally ok with what happened.

Cue his first meeting with Ignis, who eventually and with growing horror realizes he recognizes Noctis' new and underage friend from a series of artistic but pornographic photos he enjoys. Cue the messy fallout of Ignis realizing he's in possession of child porn, and knowing that Prompto was exploited in the making of it. Along with trying to see justice done, but discreetly so as not created a scandal involving the Citadel and Noctis or jeopardize the friendship between Noctis and Prompto.