Once you get your answer, keep in mind that formal balls at cons can get to rather high temperatures (maybe not as high as the more club-like dances, but still pretty high, especially depending on how many people they pack into a room), so if your friend is allowed to go in a fursuit, be sure to keep your friend's physical comfort in mind as much as possible, and take heat into consideration. (I don't overheat easily, but I've gotten a bit overheated at Fanime's formal ball before, wearing a cotton halter-style lolita dress, which isn't a particularly warm item of clothing, especially compared to a fursuit! One year when the room was fairly small and packed with dancers, I had friends who had to leave because of the heat....hopefully ColossalCon doesn't have an undersized room, but it is something to keep in mind)
Once you get your answer from con staff (who, as AshofRebirth said, are going to be the only people who would know and be able to give a concrete answer), and if your friend is allowed to wear the fursuit, maybe come up with a back-up outfit, just in case the heat is too much.
(Japan Expo 2013) Callisto Yew (Ace Attorney Investigations)
(SacAnime S2013) Fancy!Grimdark Rose Lalonde (Homestuck) ♣ Ballgown Dreamer Roxy Lalonde (Homestuck) ♠ Midnight Crew-inspired Alpha!Rose Lalonde (Homestuck)
Original design!Kanaya Maryam (Homestuck)