Ah, the fun of the Pool Party splash backgrounds! I love how much stuff is going on.
Your friends are correct that the guy with the broken noodle in PP Fiora's splash is not Twisted Fate. While, I have not seen any official confirmation from a Rioter, the evidence points to that man being Tryndamere.
Observe the Avarosan tattoo on his left arm. The Avarosan faction is not very large, and only includes one muscled sword guy, the Barbarian King himself. Also note the necklace, which can be clearly seen in Tryndamere's splash, while TF is not so accessorized.
Bonus: Check under the umbrella behind Darius' back. The white-haired woman there is almost certainly Ashe (hair is the right color/ length, plus it looks like her bikini has more Avarosan symbols). She seems less than pleased at her husband's obvious admiration for Fiora's prowess.
Cheers!
-Nixtarma