It's a relatively open secret that Laenor Velaryon is in love with his protector, Ser Joffrey Lonmouth, the Knight of Kisses. Rhaenyra seems to sense as much when she uses the metaphor of preferring duck to goose to propose an understanding between them upon their betrothal (in the book, Grand Maester Mellos says, "I do not like the taste of fish, but when fish is served, I eat it," to describe how Laenor and Rhaenyra's marriage might work).
In "House of the Dragon," Laenor and Joffrey agree that his union with Rhaenyra is the best outcome they could've hoped for, especially once the observant Joffrey notices Ser Criston struggling to watch Rhaenyra on the dance floor. Laenor and Joffrey, who go so far as to nuzzle each other in public, decide to let Criston in on their secret arrangement. They don't know the knight had asked the princess to run away with him to Essos... an offer she politely turned down to fulfill her sovereign duties (according to one source, she's less polite about it in the book). An enraged Criston beats Joffrey in front of the guests and, curiously, never faces any consequences.
Criston Cole does kill Joffrey Lonmouth in "Fire & Blood," but under much more justifiable circumstances. At a tournament following the wedding (Book Viserys loved his tourneys), Ser Harwin rode for Rhaenyra, Ser Joffrey rode for Laenor, and Ser Criston rode for Alicent. In competition, Criston smashed in Joffery's helm with his morning star and knocked him unconscious. He lived another six days, during which newlywed Laenor stayed by his side, but he never woke.
