All fights ending after the boss escape timer run out, regardless if the boss escaped or killed by another player before the time expired, is considered late. This way we don't have bunch of players sitting in a fight against dead boss for a very long time. It's recommend to finish the battle before the boss's escape.
ahh I think I can see how it could be a problem, where someone can keep the boss alive and listed for others to fight, even after the timer expires, by idling in the fight.. but no, you can just disallow anyone to initiate a fight after the timer has expired and it doesn't really matter if someone is idling in the fight, no?