Ignis and Noct are a couple, but keep it a secret. Prompto finds out and gets jealous because he's interested in Noct as well (and kind of thought they had the beginnings of a ~thing~ going because they're such close friends).
In the first moment of jealousy, he is tempted to tell on them out of the spite of his broken heart because he knows this would probably break them up. But Prompto's a good guy, and a good friend, so he doesn't (or if he already did tell someone, he does his utmost to make up for his mistake).